Rounds 1-2: Have each person take a window. If there's 3 people, have one take two of them. For the first two rounds, knife ONLY. But let them inside first. Knifing them after you let them inside will give you a possibility for Max Ammo, Double Points etc. On round two it will take two knifes to kill a zombie, so be careful!
Round 3: Use your pistols AND knife. If you are out of pistol ammo, buy a Kar-98k. DO NOT buy a Gewehr. It's a waste, unless you suck at headshots, but two shots to the body with the Kar still work. It's cheap enough anyway.
Round 4: Open the door that is up the ramp, NOT up the stairs. Have the person with the most points start. Keep moving, up the stairs, and DON'T buy any weapons. Drop into the random weapons area, and turn the power on (Not the wall. The power that drops the bridges. This is very important that the bridges are dropped.). Open up the doors to Teleporter C. This is the area you will be in for the whole game.
Again, DON'T buy any weapons, until you get into the Teleporter C room. Have everyone buy an STG off the wall. This will be the weapon you use for the majority of the time. Everyone should walk up the stairs to the catwalk. Have one person go to the window and keep a zombie alive. Have one person watch the stairs, killing the zombies as they come up. Have another at the windows behind the stairs, shooting zombies as they come up. And then have another helping the stairs person.
Since a zombie is kept alive, everyone who has 2500 points needs to go buy juggernaut. You will NEED this. Buy it before anything else.
Once everyone is in their places, kill the last zombie.
Round 6: As usual have one person keep a zombie alive, everyone do their jobs. As needed, go buy STG ammo when possible.
When there's one zombie left, everyone EXCEPT the window person needs to go link all the teleporters to open up the Pack-A-Punch. Once Pack-A-Punch is open and everyone is back in their places, kill the zombie.
Rounds 7-10: Do the usual. Once there is one zombie left, people who have enough need to go upgrade their STG to the Spatz-447 with the Pack-a-Punch. With Pack-A-Punch, an MG is NOT needed like in the first 3 zombies.
By round 10 everyone should have an upgraded STG.
Rounds 10-15: It starts getting REALLY hard now. Whenever one zombie is left, everyone needs to get Speed Cola and Quick Revive. Also, definitely try the random box. Monkey Bombs are CRUCIAL. If you don't have them, you might not get very far. So try till you get em!
Recommended Upgradeable Weapons:
STG
PPSH
Ray Gun
WunderWaffe DG-2
Colt (Might sound weird, but it's a life saver. It shoots impact-exploding grenades! You might wanna save it.)
Rounds 16-20: This is when it starts getting INSANE. Trust me. Infact, at 16, dogs start coming at the same time as zombies! If you see Insta-Kill or Max Ammo, GET IT. If you see a Hammer, DON'T. It's useless. Double Points is helpful, but not worth risking your life. Nukes should only be obtained at the beginnings of rounds, as you need to keep a zombie alive.
Remember, if you need STG ammo, use a monkey bomb. It gives you enough time to buy ammo.
Have the people with the best weapons at the stairs. A ray gun is recommended for the windows overlooking the map. For the person who is keeping one alive, use ONLY bullet weapons. No explosives or energy weapons.
By 20, you will CONSTANTLY need ammo. This is why the STG is the best weapon for this, and why Monkey Bombs are crucial.
Rounds 21-28: THIS. IS. INSANELY. HARD. Trust me. By now, get nukes WHENEVER you see it. You will need them.
Basically the same strategy. Take a LOT of time while a zombie is alive to resupply on ammo, weapons, perks etc. Not much of a difference, but insanely harder.
Round 29-Up: I imagine that it's the same strategy for the rest of the game. It's a great strategy. However I haven't made it past 27, so I will update this guide when I beat my record.
Tips:
1. The random box is your friend. Only in higher rounds, though. The STG, however is your BEST friend.
2. A ray gun is very helpful. Upgrade it and you have an amazing weapon. Aim it at the bottom of the stairs. Great strategy.
3. Juggernaut is REQUIRED. I have no idea how the hell people do the Perkaholics Anonymous achievement.
4. Bowie Knife isn't all that great. Good for keeping one alive though.
5. Use the teleporter right behind you as a last resort. But only if you need it.
6. Buy betties, they will save your life a lot. Their also great for dog rounds. Place them on the catwalk, always. Don't put them in front of the window where one is kept alive.
That's all for now, but I'll update it as I do better. Thanks for reading!