Alright since my Unreal Tournament 3 Achievement Guide seemed to be a hit back in the day, I thought I'd do another one for Red Dead Redemption, since it is a fantastic game and has a lot of difficult (I think) achievements.
If you're worried about looking at the Secret Achievements, they contain no spoilers to the story line but I'll but them in a spoiler box anyway.
So here we go:
High Roller (10G) - Win over 2000 chips in a hand of Poker
-The best place to do this is Blackwater, as it has the highest-stakes Poker games, so the more chance you can get the achievement. Simply win a hand when there is 2000$ or more in the pot. Might take a while if you're bad a poker.
*Thanks to BluntedGrinch* You can also get this fairly easy by playing the mission "Lucky In Love" and going all in, and hoping another player does as well. Then win the hand, and achievement.
No Dice (10G) - Complete a game of Liar's Dice without losing a single die.
-Go to any Liar's Dice station and start a game of dice. To make sure you never lose a die, always make a bet you know is true (I.e., if you have one three, bet one three because you KNOW it is true). The AI will normally call you on bets and you can slowly whittle their dice away without losing yours.
What About Hand Grenades? (10G) - Get a ringer in a game of Horseshoes.
-Go to any game of Horseshoes and start playing. Press LT to aim, and get the post right in the middle of the two prongs of your horseshoe, right to the left of your index finger. The its just timing, throwing the horseshoe right when the bar crosses the white line, and you should get it.
Austin Overpowered (25G) - Complete Twin Rocks, Pike's Basin, and Gaptooth Breach Gang Hideouts in Single Player.
-All the 7 single player hideouts will be located on your map. To complete a hideout, simply start the mission by talking to the yellow dot on your map, and then kill all the enemies. Some require a little extra work besides eliminating the enemies, but it shouldn't be hard.
For the MP hideout achievement, simply do all 7 hideouts in one session (not quitting Free Roam/disconnecting). More people you have in your posse, the easier this will be.
Evil Spirits (25G) - Complete Tumbleweed and Tesoro Azul Gang Hideouts in Single Player.
-See "Austin Overpowered"
Instinto Asesino (25G) - Complete Fort Mercer and Nosalida Gang Hideouts in Single Player.
-See "Austin Overpowered"
Fightin' Around the World (5G) - Knock someone out in melee in every saloon in the game in Single Player.
-There are seven saloons in the game:
New Austin:
Armadillo, Thieves' Landing, RathSkeller Fork
Nuevo Paraiso:
Escalera, Chuparosa, Casa Madrugada
West Elizabeth:
BlackWater
Simply go into a saloon and melee a person until they are knocked out.
Strange Things are Afoot (10G) - Complete a task for a Stranger.
-Very easy. Simply go up to a "?" on your map and do the mission.
People are Still Strange (25G) - Complete 15 tasks for Strangers.
-Do fifteen of the "?" missions on your map.
Buckin' Awesome (10G) - Break the Kentucky Saddler, the American Standardbred, and the Hungarian Half-bred.
-This achievement is a little tricky. Once you have obtained the lasso, you must lasso these three horses and "break" them (the mini game where you have to balance and eventually gain control of the horse). Location for each:
Kentucky Saddler (Golden/Yellow horse) - Near Macfarlane's Ranch.
American Standardbred (All black (the coolest, IMO)) - Near Beecher's Hope.
Hungarian Half Bred (White w/dark face) - Between Escalera and Punta Orugullo.
Clemency Pays (10G) - Capture a bounty alive.
-Easy one. When a bounty appears (a skull on your map) accept it and travel to his location. I would advise killing all but the bounty (the skull), and then shooting him in the legs/knees so he can't run. Then simply run up and lasso him, but him on the back of your horse, and return him to the jail where you accepted the bounty.
Exquisite Taste (10G) - Purchase a rare weapon from a gunsmith.
-There are five rare guns in the game:
Mauser Pistol ($800) - Blackwater
LeMat Revolver ($1250) - Escalera (must finish "An Appointed Time" Mission first)
Evans Repeater ($1000) - Blackwater
Semi-Auto Shotgun ($1100) - Escalera
Carcano Sniper Rifle ($1100) - Blackwater
Just buy one of these and you'll get the achievement.
Bearly Legal (5G) - Kill and skin 18 grizzly bears.
-Exactly as the achievement says. Bears are mostly found in the area between Bearclaw Camp and Tall Trees on your map in West Elizabeth.
He Cleans Up Well! (10G) - Obtain the Elegant Suit.
-After a point in the story this outfit will become available, and all you have to do is purchase it in the tailer in Thieves' Landing.
More than a Fistful (10G) - Earn $10,000 in Single Player.
-This one is a bit hard to get. You'll get this eventually if you keep playing after the story, but it's difficult. One good way is to hunt bear and buffalo. You can get around $100 a buffalo (all the pieces) is you sell them in Mexico, and around $80 a bear (all the pieces) in Mexico as well. This way takes awhile, but is probably your fastest way of getting money.
This achievement is cumulative, so you don't ever have to have a total of $10,000 on your person.
Frontiersman (20G) - Obtain Legendary rank in any Single Player Ambient Challenge.
-There are four challenges in Single Player, Survivalist, Hunting, Sharpshooter, and the Treasure Hunt. All four have ten levels, get to level 10 on any of the four for the achievement.
The Gunslinger (5G) - Score a headshot on any enemy using Expert targeting mode.
-Very easy 5 points. You can change the targeting mode under the settings in the in-game pause menu. Get near an enemy, switch to Expert, pop his dome and boom, achievement.
Man of Honor/Chivalry's Dead (25G) - Attain highest Fame rank and either highest Honor rank or lowest Honor rank.
-Like the achievement says. Honor is rather easy, don't do anything bad and you'll get full high honor about half way through the game. For fame, you pretty much have to complete every story mission and stranger mission and win some duels. It take much longer, so work hard.
Gold Medal (25G) - Earn a Gold Medal Rank for a combat mission in Single Player.
-Gold medals are awarded for how well you preform on a mission. You have to finish it quite fast (cutscenes count as time, so I advise doing this after you beat the main story (you can replay missions) so you can skip the cutscenes), and kill most of the enemies and have a decent accuracy rating as well as head shot count. A bit difficult, but keep trying on some of the shorter ones and it shouldn't be too hard.
*Thanks to kslay* A good mission to do this achievement is "Political Realities in Armadillo."
On the Trail of de Vaca (10G) - Uncover every location on the map in Single Player.
-There are 97 map locations in the game, I have included a map below. You can also just buy all the locations from a store, if you have some extra money.
Friends In High Places (10G) - Use a pardon letter with more than $5000 bounty in Single Player.
-This one is a lot harder than it sounds. Getting up to a $5000 bounty without dying can take some serious time and skill. I advise going into a populated area (Blackwater, Escalera maybe) and just kill EVERYONE. Take cover a lot, and work your way up to 5K. Then hop on your horse and get the wanted to fade, and use a pardon letter to clear it all.
Redeemed (100G) - Attain 100% in the Single Player Completion stat.
-No doubt the hardest achievement in the game, it'll take you a while. Here are the requirements for getting 100%:
-Complete all 57 story missions
-Collect all 5 rare weapons
-Collect the 9 main outfits
-Buy All 13 properties
-Work all 5 jobs
-Beat the 7 Gang Hideoutes
-Reach level 10 in all four Single Player Challenges
-Complete all the bounties
-Do all 18 Stranger Missions
-Play all 6 Mini games
-Discover all 97 map locations.
I'll include a more detailed guide for the 100% in my next post.
Mowing Them Down (20G) - Kill 500 enemies with a mounted weapon in any game mode.
-There are three mounted guns in the game: The Cannon, Gatling Gun, and Browning Machine gun. I would advise playing the missions with these guns over and over until you get the achievement. This can be done in SP, MP, and Free Roam.
In a Hall of Bullets (20G) - Kill 500 enemies with any pistol or revolver in any game mode.
-This achievement should come naturally if you use the pistol a lot, you should get it near the end of the game. But if you're like me, a rifle man, I would switch to the pistol immediately after you get the Rifle achievement and use a pistol for the rest of the game, as well as complete the Hideouts with one. This can be done in SP, MP, and Free Roam.
Long Arm of Marston (20G) - Kill 500 enemies with any rifle, repeater, or shotgun in any game mode.
-This achievement should come naturally if you use the rifles a lot, you should get it near the end of the game. But if you used the pistol a lot, I would switch to the rifle immediately after you get the Pistol achievement and use a rifle for the rest of the game, as well as complete the Hideouts with one. This can be done in SP, MP, and Free Roam.
Bullseye (20G) - Get 250 headshots in any game mode.
-You should get this one naturally throughout the story, but it can be done in SP, MP, and Free Roam.
Unnatural Selection (20G) - Kill one of every animal species in the game in any game mode.
-Another difficult one. Mainly because it is still kind of unknown how many animals there are in the game, and which count towards the achievement. But according to the wiki, there are 36 animals that need to be killed for the achievement:
Animals:
American Buffalo
Armadillos
Bats
Beavers
Bighorn Sheep
Boars
Bobcats
Cattle
Bulls
Chickens
Coyotes
Cougars
Crows
Deer
Bucks
Dogs
Donkeys
Ducks
Eagles
Elk
Foxes
Goats
Grizzly Bears
Hawks
Horses
Mules
Owls
Pigs
Rabbits
Raccoons
Rattlesnakes
Seagulls
Song Birds
Skunks
Vultures
Wolves
I will include a detailed list of the animals locations in my next post.
Have Gun Will Travel (20G) - Complete all Gang Hideouts in a single public Free Roam session.
-See "Austin Overpowered"
Slow on the Draw (10G) - Get 10 assists in a single Gang Hideout in a public Free Roam session.
-You'll know you get an assist if a +2 appears on your screen. Easiest way to do this is to start a Hideout and run around and shoot everyone once or twice with your pistol, then have your posse members finish them off. Once again, more people in your posse the easier.
Hit the Trail (10G) - Get from Blackwater to Escalera before sundown in a public Free Roam session.
-Exactly what it says. I advise using a teleport wheel to transport to Blackwater so you make sure the game knows you're there, then ride all the way to Escalera without stopping. The trip takes about 5 hours (in game time) with the worst horse you can get in Free Roam, and sunset is around 6pm, so I advise starting no later than 9am just to be safe.
Posse Up! (10G) - Create a posse and get the maximum number of members.
-Go into a public Free Roam and invite all your friends. You need 8 people total (including you) in your posse to get this achievement.
*Thanks to Meta* You must be the leader of the posse to get this achievement.
The Quick and Everyone Else… (20G) - Be the top scoring player in any three consecutive FFA games in public matches.
-Like the achievement says. Get good at the game, win some matches in a row.
How the West Was Won (20G) - Reach the top rank for multiplayer experience.
-Goes up to level 50. This actually doesn't seem like it will take that long, as I've only logged in about 1 hour to the multiplayer and am already level 8.
Go Team! (20G) - Be on the winning team for four consecutive victories in any team based game in public matches.
-Like the achievements says. Get a good team together and win some matches in a row.
Most Wanted (10G) - Become a Public Enemy for 10 minutes and escape alive in a public Free Roam session.
-In order to become a public enemy you need to have a bounty of at least $1000 dollars. Then ride around for 10 minutes without your Wanted meter fading, and then escape alive for the achievement.
Red Dead Rockstar (10G) - Kill a Rockstar or someone with this achievement in a public multiplayer match.
-It's a viral achievement, so just play the multiplayer (free roam and regular matches) and you should get it soon. I got it the first match I played.
Map of the Game with all the locations: http://i817.photobucket.com/albums/zz97/GODCHILD1020/Untitled.png
Alright that's it, hope it helps you all. Thanks to: GregoryThomass, Tha MasterChief, and the Red Dead wiki.
I'll include a more detailed guide for the 100% achievement and the animal achievement below.
If you're worried about looking at the Secret Achievements, they contain no spoilers to the story line but I'll but them in a spoiler box anyway.
So here we go:
High Roller (10G) - Win over 2000 chips in a hand of Poker
-The best place to do this is Blackwater, as it has the highest-stakes Poker games, so the more chance you can get the achievement. Simply win a hand when there is 2000$ or more in the pot. Might take a while if you're bad a poker.
*Thanks to BluntedGrinch* You can also get this fairly easy by playing the mission "Lucky In Love" and going all in, and hoping another player does as well. Then win the hand, and achievement.
No Dice (10G) - Complete a game of Liar's Dice without losing a single die.
-Go to any Liar's Dice station and start a game of dice. To make sure you never lose a die, always make a bet you know is true (I.e., if you have one three, bet one three because you KNOW it is true). The AI will normally call you on bets and you can slowly whittle their dice away without losing yours.
What About Hand Grenades? (10G) - Get a ringer in a game of Horseshoes.
-Go to any game of Horseshoes and start playing. Press LT to aim, and get the post right in the middle of the two prongs of your horseshoe, right to the left of your index finger. The its just timing, throwing the horseshoe right when the bar crosses the white line, and you should get it.
Austin Overpowered (25G) - Complete Twin Rocks, Pike's Basin, and Gaptooth Breach Gang Hideouts in Single Player.
-All the 7 single player hideouts will be located on your map. To complete a hideout, simply start the mission by talking to the yellow dot on your map, and then kill all the enemies. Some require a little extra work besides eliminating the enemies, but it shouldn't be hard.
For the MP hideout achievement, simply do all 7 hideouts in one session (not quitting Free Roam/disconnecting). More people you have in your posse, the easier this will be.
Evil Spirits (25G) - Complete Tumbleweed and Tesoro Azul Gang Hideouts in Single Player.
-See "Austin Overpowered"
Instinto Asesino (25G) - Complete Fort Mercer and Nosalida Gang Hideouts in Single Player.
-See "Austin Overpowered"
Fightin' Around the World (5G) - Knock someone out in melee in every saloon in the game in Single Player.
-There are seven saloons in the game:
New Austin:
Armadillo, Thieves' Landing, RathSkeller Fork
Nuevo Paraiso:
Escalera, Chuparosa, Casa Madrugada
West Elizabeth:
BlackWater
Simply go into a saloon and melee a person until they are knocked out.
Strange Things are Afoot (10G) - Complete a task for a Stranger.
-Very easy. Simply go up to a "?" on your map and do the mission.
People are Still Strange (25G) - Complete 15 tasks for Strangers.
-Do fifteen of the "?" missions on your map.
Buckin' Awesome (10G) - Break the Kentucky Saddler, the American Standardbred, and the Hungarian Half-bred.
-This achievement is a little tricky. Once you have obtained the lasso, you must lasso these three horses and "break" them (the mini game where you have to balance and eventually gain control of the horse). Location for each:
Kentucky Saddler (Golden/Yellow horse) - Near Macfarlane's Ranch.
American Standardbred (All black (the coolest, IMO)) - Near Beecher's Hope.
Hungarian Half Bred (White w/dark face) - Between Escalera and Punta Orugullo.
Clemency Pays (10G) - Capture a bounty alive.
-Easy one. When a bounty appears (a skull on your map) accept it and travel to his location. I would advise killing all but the bounty (the skull), and then shooting him in the legs/knees so he can't run. Then simply run up and lasso him, but him on the back of your horse, and return him to the jail where you accepted the bounty.
Exquisite Taste (10G) - Purchase a rare weapon from a gunsmith.
-There are five rare guns in the game:
Mauser Pistol ($800) - Blackwater
LeMat Revolver ($1250) - Escalera (must finish "An Appointed Time" Mission first)
Evans Repeater ($1000) - Blackwater
Semi-Auto Shotgun ($1100) - Escalera
Carcano Sniper Rifle ($1100) - Blackwater
Just buy one of these and you'll get the achievement.
Bearly Legal (5G) - Kill and skin 18 grizzly bears.
-Exactly as the achievement says. Bears are mostly found in the area between Bearclaw Camp and Tall Trees on your map in West Elizabeth.
He Cleans Up Well! (10G) - Obtain the Elegant Suit.
-After a point in the story this outfit will become available, and all you have to do is purchase it in the tailer in Thieves' Landing.
More than a Fistful (10G) - Earn $10,000 in Single Player.
-This one is a bit hard to get. You'll get this eventually if you keep playing after the story, but it's difficult. One good way is to hunt bear and buffalo. You can get around $100 a buffalo (all the pieces) is you sell them in Mexico, and around $80 a bear (all the pieces) in Mexico as well. This way takes awhile, but is probably your fastest way of getting money.
This achievement is cumulative, so you don't ever have to have a total of $10,000 on your person.
Frontiersman (20G) - Obtain Legendary rank in any Single Player Ambient Challenge.
-There are four challenges in Single Player, Survivalist, Hunting, Sharpshooter, and the Treasure Hunt. All four have ten levels, get to level 10 on any of the four for the achievement.
The Gunslinger (5G) - Score a headshot on any enemy using Expert targeting mode.
-Very easy 5 points. You can change the targeting mode under the settings in the in-game pause menu. Get near an enemy, switch to Expert, pop his dome and boom, achievement.
Man of Honor/Chivalry's Dead (25G) - Attain highest Fame rank and either highest Honor rank or lowest Honor rank.
-Like the achievement says. Honor is rather easy, don't do anything bad and you'll get full high honor about half way through the game. For fame, you pretty much have to complete every story mission and stranger mission and win some duels. It take much longer, so work hard.
Gold Medal (25G) - Earn a Gold Medal Rank for a combat mission in Single Player.
-Gold medals are awarded for how well you preform on a mission. You have to finish it quite fast (cutscenes count as time, so I advise doing this after you beat the main story (you can replay missions) so you can skip the cutscenes), and kill most of the enemies and have a decent accuracy rating as well as head shot count. A bit difficult, but keep trying on some of the shorter ones and it shouldn't be too hard.
*Thanks to kslay* A good mission to do this achievement is "Political Realities in Armadillo."
On the Trail of de Vaca (10G) - Uncover every location on the map in Single Player.
-There are 97 map locations in the game, I have included a map below. You can also just buy all the locations from a store, if you have some extra money.
Friends In High Places (10G) - Use a pardon letter with more than $5000 bounty in Single Player.
-This one is a lot harder than it sounds. Getting up to a $5000 bounty without dying can take some serious time and skill. I advise going into a populated area (Blackwater, Escalera maybe) and just kill EVERYONE. Take cover a lot, and work your way up to 5K. Then hop on your horse and get the wanted to fade, and use a pardon letter to clear it all.
Redeemed (100G) - Attain 100% in the Single Player Completion stat.
-No doubt the hardest achievement in the game, it'll take you a while. Here are the requirements for getting 100%:
-Complete all 57 story missions
-Collect all 5 rare weapons
-Collect the 9 main outfits
-Buy All 13 properties
-Work all 5 jobs
-Beat the 7 Gang Hideoutes
-Reach level 10 in all four Single Player Challenges
-Complete all the bounties
-Do all 18 Stranger Missions
-Play all 6 Mini games
-Discover all 97 map locations.
I'll include a more detailed guide for the 100% in my next post.
Mowing Them Down (20G) - Kill 500 enemies with a mounted weapon in any game mode.
-There are three mounted guns in the game: The Cannon, Gatling Gun, and Browning Machine gun. I would advise playing the missions with these guns over and over until you get the achievement. This can be done in SP, MP, and Free Roam.
In a Hall of Bullets (20G) - Kill 500 enemies with any pistol or revolver in any game mode.
-This achievement should come naturally if you use the pistol a lot, you should get it near the end of the game. But if you're like me, a rifle man, I would switch to the pistol immediately after you get the Rifle achievement and use a pistol for the rest of the game, as well as complete the Hideouts with one. This can be done in SP, MP, and Free Roam.
Long Arm of Marston (20G) - Kill 500 enemies with any rifle, repeater, or shotgun in any game mode.
-This achievement should come naturally if you use the rifles a lot, you should get it near the end of the game. But if you used the pistol a lot, I would switch to the rifle immediately after you get the Pistol achievement and use a rifle for the rest of the game, as well as complete the Hideouts with one. This can be done in SP, MP, and Free Roam.
Bullseye (20G) - Get 250 headshots in any game mode.
-You should get this one naturally throughout the story, but it can be done in SP, MP, and Free Roam.
Unnatural Selection (20G) - Kill one of every animal species in the game in any game mode.
-Another difficult one. Mainly because it is still kind of unknown how many animals there are in the game, and which count towards the achievement. But according to the wiki, there are 36 animals that need to be killed for the achievement:
Animals:
American Buffalo
Armadillos
Bats
Beavers
Bighorn Sheep
Boars
Bobcats
Cattle
Bulls
Chickens
Coyotes
Cougars
Crows
Deer
Bucks
Dogs
Donkeys
Ducks
Eagles
Elk
Foxes
Goats
Grizzly Bears
Hawks
Horses
Mules
Owls
Pigs
Rabbits
Raccoons
Rattlesnakes
Seagulls
Song Birds
Skunks
Vultures
Wolves
I will include a detailed list of the animals locations in my next post.
Have Gun Will Travel (20G) - Complete all Gang Hideouts in a single public Free Roam session.
-See "Austin Overpowered"
Slow on the Draw (10G) - Get 10 assists in a single Gang Hideout in a public Free Roam session.
-You'll know you get an assist if a +2 appears on your screen. Easiest way to do this is to start a Hideout and run around and shoot everyone once or twice with your pistol, then have your posse members finish them off. Once again, more people in your posse the easier.
Hit the Trail (10G) - Get from Blackwater to Escalera before sundown in a public Free Roam session.
-Exactly what it says. I advise using a teleport wheel to transport to Blackwater so you make sure the game knows you're there, then ride all the way to Escalera without stopping. The trip takes about 5 hours (in game time) with the worst horse you can get in Free Roam, and sunset is around 6pm, so I advise starting no later than 9am just to be safe.
Posse Up! (10G) - Create a posse and get the maximum number of members.
-Go into a public Free Roam and invite all your friends. You need 8 people total (including you) in your posse to get this achievement.
*Thanks to Meta* You must be the leader of the posse to get this achievement.
The Quick and Everyone Else… (20G) - Be the top scoring player in any three consecutive FFA games in public matches.
-Like the achievement says. Get good at the game, win some matches in a row.
How the West Was Won (20G) - Reach the top rank for multiplayer experience.
-Goes up to level 50. This actually doesn't seem like it will take that long, as I've only logged in about 1 hour to the multiplayer and am already level 8.
Go Team! (20G) - Be on the winning team for four consecutive victories in any team based game in public matches.
-Like the achievements says. Get a good team together and win some matches in a row.
Most Wanted (10G) - Become a Public Enemy for 10 minutes and escape alive in a public Free Roam session.
-In order to become a public enemy you need to have a bounty of at least $1000 dollars. Then ride around for 10 minutes without your Wanted meter fading, and then escape alive for the achievement.
Red Dead Rockstar (10G) - Kill a Rockstar or someone with this achievement in a public multiplayer match.
-It's a viral achievement, so just play the multiplayer (free roam and regular matches) and you should get it soon. I got it the first match I played.
Secret Achievements
Spoiler: Click here to toggle spoiler
That Government Boy (10G) - Complete "Exodus in America."
-Complete the Single Player Mission "Exodus in America."
Land of Opportunity (30G) - Complete "The Assault on Fort Mercer."
-Complete the Single Player Mission "The Assault on Fort Mercer."
Sons of Mexico (40G) - Complete "The Gates of El Presidio."
-Complete the Single Player Mission "The Gates of El Presidio."
No More Fancy Words (20G) - Complete "An Appointed Time."
-Complete the Single Player Mission "An Appointed Time."
A Savage Soul (10G) - Complete "At Home with Dutch."
-Complete the Single Player Mission "At Home with Dutch."
The Benefits of Civilization (90G) - Complete "And the Truth Will Set You Free."
-Complete the Single Player Mission "And the Truth Will Set You Free."
Into the Sunset (100G) - Complete "The Last Enemy That Shall Be Destroyed."
-Complete the Single Player Mission "The Last Enemy That Shall Be Destroyed."
Nurture or Nature? (50G) - Complete "Remember My Family"
-Complete the side quest/stranger mission "Remember My Family."
Dastardly (5G) - Place a hogtied woman on the train tracks, and witness her death by train.
-Once you obtain the lasso, throw it at a woman and hogtie her. Then look on your map for where the train is, and travel to a decent amount in front of it. Place the woman on the tracks and keep the camera trained on her body until she gets run over 
Spurred to Victory (10G) - Complete 20 story missions without switching to a new horse at a hitching post.
-As long as you don't switch horses from the one given you obtain in the mission with Bonnie, you should get this one easily. If your horse dies or you got a new one, just make sure to complete twenty missions that involve you using your horse without getting a new horse.
Heading South on a White Bronco (5G) - Evade the US Marshals while riding the Hungarian Half-Bred horse in Single Player.
-After purchasing/obtaining the Hungarian Half-Bred, rack up a $1000 bounty. This will cause the Marshals to appear (you have to be in New Austin/West Elizabeth to do this, as they are the US Marshals), and then simply kill them all or outrun them so your wanted meter fades.
Manifest Destiny (5G) - Kill the last buffalo in the Great Plains in Single Player.
-If you go to the Great Plains area (West of Blackwater), there'll be a group of about twenty buffalo. These are all the buffalo in the entire game, so just kill them all for the achievement. No more respawn, so make sure you skin all of them in order to get the most money so you can get your "More Than a Fistful" achievement.
Map of the Game with all the locations: http://i817.photobucket.com/albums/zz97/GODCHILD1020/Untitled.png
Alright that's it, hope it helps you all. Thanks to: GregoryThomass, Tha MasterChief, and the Red Dead wiki.
I'll include a more detailed guide for the 100% achievement and the animal achievement below.
