Complete any level
Start at level 1-1 and complete i. You can set it at the easiest difficulty and run right through the level in under 20 seconds. From the beginning run straight forward through the hall and open the door, run around to the right of the stage in the middle of the room to the next, then follow the zig zag walk way to the next door. Open and run past the enemy to the next door and exit. Take a look at my walkthrough for 4 easy achievements in one level under HOARDER(Note: Remember that you can sprint with the Left trigger).
SCOUT
Find a secret area of a map
Can be done on the easiest difficulty. In level 1-1 get to the area with the zig zag walkway. Walk across to where the next door is and on the right wall before the door there is a secret panel. Press A near the discolored wall to find the secret door and go in. Theres a small hallway that leads into a wide open area with a 200% armor in the middle of a poison pool. You wont get the achievement until you complete the level. See the HOARDER achievement for a walkthrough on getting 4 easy achievements in one level.
CLEAN SLATE
Kill all monsters in level on 'Hurt Me Plenty' or higher
Easily done in level 1-1 on Hurt Me Plenty. Dont bother going in the room on the left unless you want the armor. Continue through the level to the first door. Kill the 2 enemies in the first room then go to the next. Kill the 1 enemy in the hall before the zig zag walkway room and the guy at the end of the walkway. Theres a monster up on the wall in the corner that you need to take care of as well. Continue through the next door. Kill the monster in there and finish the level for the achievement.
HOARDER
Find all items in level
Can be done on easiest setting but heres a walkthrough for getting 4 achievements in one run through level 1-1. Set the difficulty to Hurt Me Plenty and start the game on Knee Deep in the Dead (Episode one aka level 1-1). Starting off look to the right for a blue health bonus, then turn around and grab the next one to the left of where you started. Theres also 2 armor bonus items on the opposite wall. Now go into the room with the stairs to the left of where you start. Theres 4 health bonuses and 4 armor bonuses along with a 100% armor bonus. Now leave and continue through the level. Kill the 2 enemies in here and pick up their clips along with the 6 armor bonuses in the room. Continue on and kill the next enemy in the hall. Grab his clip and the 2 blue health bonuses and move on. Kill the monster in the top corner and the soldier at the end. Grab his clip and the 2 shotgun ammo clips. Go through the secret panel before the next door (See SCOUT achievement for details) and pick up the box of bullets, health box, and box of shotgun bullets. NOTE: If you cant pick up a health box at any point during this level just jump in the green poison water until your health is below 100%. Now keep going through the hall to the open area and grab the 200% armor bonus in the middle of the poison pool then back track to the secret door and out. Turn right and go through the next door and kill the monster. In this next room there is 1 armor bonus, 2 clips, 4 health bonuses and 1 health box. Now go into the exit room and grab the 4 armor bonuses and mini health pack but DO NOT EXIT!! Youve killed every enemy but there are 2 more secret areas to find. Back track to the zig zag walk way room and where you shot the monster in the corner, the wall has been lowered. Go in to find a shotgun, box of shotgun bullets, and a health box. Now for the last secret area. This one is a little tricky so pay attention. In the secret room you were just in, turn around and look out the door. You'll be looking at the zig zag room. Now you see the doorway on the other side? Hold in the left trigger and sprint to it (dont bother zigging and zagging). Right when you get to the FIRST stair, turn completely around and SPRINT back into the secret room where you came from. On the back wall in the left corner, a small piece of wall has lowered. You need to run fast and get onto it before it goes back up. There are 8 armor bonuses and 1 box of shotgun shells. If you have max bullets just shoot one off to pick up the box of shells. Now jump down, run to the next room and to the exit. 4 achievements in one run and it only takes 5 minutes!!