Call of Duty: World at War

The trick to Perkaholics Anonymous is firepower- lots and lots of firepower. You are going to want as many upgraded Ray Guns as you can get your hands on, as well as an upgraded Wonder Waffle. If you are having bad luck at the box and can't get the Ray Gun, then the Trench Gun upgrades into a powerful shotgun (the "Gut Shot") that will remain a 1-shot kill well past level 20.

Once you've got some serious firepower, go into the room behind where you turn on the power (the room where you get the bowie knife). By the teleporter is a dead-end hallway with a window halfway down. Have one person (someone without a ray gun) crouch next to the window, while everyone else goes further down the hall to provide support. The guy at the window is responsible for any zombies at the window, and everyone else is responsible for making sure no zombies sneak up on the guy at the window and kill him. It would help if the guy holding the window had Juggernog, but it's not necessary- when I got my Perksaholic, it was in a 3-player game, and none of us had any perks.

Have the guys in the back unload ammo as fast as they need to to ensure that nobody touches the window guy. When you start running low on ammo, tell the other support guys to reload so that, by the time you're out of ammo, they should have a fresh clip and you can reload in peace. While it's tempting to look out the gaps to the left and shoot zombies there, if they drop any powerups you won't be able to collect them, so I wouldn't recommend it. If someone goes down, have someone toss a monkey bomb out the gaps to the left and then revive while all the zombies are distracted. For dog rounds, be quick on the trigger to kill them all at the bottom of the stairs, but don't worry about using your Ray Gun if one gets close; the upgraded Ray Gun's blast deals much less damage to you. You can still kill yourselves by firing 4-5 close range blasts in rapid succession, but a single shot of the upgraded Ray Gun will never kill you, no matter how close you stand to the blast. After that, it's just a matter of time before you hit round 20. As I said, I made it with 3 players (one Ray Gun, one Wonder Waffle/MG42 combo, one Trench Gun, all of them upgraded) without a single down. It seems like a tough achievement, but the upgraded ray gun makes it really easy.

For Solo or Pair play:

Pistol/Knife for rounds 1 & 2

Get the Thompson in round 3 and then clear the door up the stairs so you can hold out by where you turn the power on. Do not open the door behind you until round 6, 7, or 8 when you are ready to link the teleporters. Hopefully you also have the Monkey Bomb which can be a life saver.

Once all of the teleporter doors are open, the map gets really difficult so circling the map similar to the Shi No Numa Big Baller method will be our goal.

Begin each round up by the electric bridge so that you then fall down into the power room/holdout room. Go straight through the door and around the corner, heading towards the Fly Trap. Now go through the door into the starting room and carefully work your way to the Thompson room then up the stairs. Turn around once standing next to the bridge and unload on the zombie pack for a few seconds before dropping down and starting another lap.

For the dog rounds, the teleporter room above the Thompson room is a decent place to hold out. Good luck!

Everything above this solution is correct, however you ARE able to die and be revived to get this achievement. You and your partners all have to reach level 20, they can also have perks if they so choose. Firepower works, Upgraded Ray Guns and Wonderwaffe's are always good. MG42; PP5h-51; and Brownings are solid backup options - upgraded of course.

A really good spot to hold out until Round 20 is the catwalk in the Teleporter room, by the Bowie Knife. It's best to do this with four people in order to maximize firepower.

The setup you want to have is one guy at the window outside (who makes sure at least one zombie is alive so that everyone else can get upgrades and weapons), two guys at the top of the stairs (typically with upgraded MG42s or Brownings) and one person in the middle of the catwalk to cover the two guys on the stairs if they need to reload.

If you are going specifically for this achievement, you either want to be the guy at the window or the guy in the middle of the catwalk (the guys on the stairs might want Jugger-nog or Speed Cola). If on the catwalk, you'll probably want and upgraded Colt or Ray Gun. If by the window, you may want an upgraded Ray Gun or FG42.

Hope this helps.

The best way to do this achievement on solo is to hold up in the power area; where the box first appears. On your way to this area make sure you grab the bouncing betty's and keep placing them whenever you get the chance. Holding in this area can be done without monkey bombs, but it is a lot easier with monkey bombs. Anyways, once you are in this area, hit the box until you have your desired weapons, (I got lucky by getting an lmg, ppsh, and monkey bombs on my first 4 hits) so you simply hold here and don't open the door behind you. This place is so good because the zombies will only come from in front of you, and the lighting trap just so happens to be right in front of the main area the zombies will come from. So you just pop the lightning trap whenever you need to, and the only zombies you have to worry about will be coming out of the lonely window to the left. Throw monkey bombs when needed if available.

There is a cool little glitch in der riese that hasnt been patched as of May 11th 2012..... and it helps for this achievement. In the quick revive room there is a hole in the floor. One person jumps in the hole and crouches in the corner opposite the giant chunk missing from the wall (a great viewspot outside) the other person stands right next to the support beam on the wall directly above the hole. if it is done properly you will notice that the zombies run back and forth outside. the person who jumped in the hole will not face any zombies as long as he/she stays crouched in the corner. the person above in the quick revive room will only have to guard the window in the room. just play like this until round twenty. an easier alternative to catwalk

NOTE: The person on top must be careful not to back up too much so not to fall in the hole. if you do fall it is no big deal you just must face the horde legit again. Also this glitch does not work on dogs rounds so when dogs spawn the person on top can jump down then run back up after

Just a video i made that might help

NOTE: I forgot to mention, by covering the window in the quick revive room i dont mean to stand by it. THE MOST ESSENTIAL STEP in this glitch is that the person in the hole is crouching in the correct corner. and the person on top is standing almost directly above against the wall by the beam... once there is a healthy build up of zombies the person on bottom can shoot like crazy through the chunk in the wall to clear the round. Wunderwaffe DG-2 helps alot for this

EDIT: Still works as of July 17th, 2015

I did solo it a few minutes ago...

It's not that hard, sure i needed a few tries and died on the higher rounds (17+).
All those guides are good, but way to complicated or forcing you to glitch this out.

The only things you need is a thompson and a 1897 Shotgun, of course all teleporters activated and for the higher rounds punched and betties for safety.
Not needed but as security: the bowie knife and monkeys.

Leave a crawler if you need to link a teleporter, i had them all linked in round 8.
After that it's just this simple solution:

EVERY ROUND place a betty on the metal bridge and one at the entrance to the room opposite.Then walk over to the ledge opposite the root beer perk.
Zombies will come and just kill them until you can't hold it any longer then jump down and throw a grenade against the wall to your left and run through the gap, when you see speed cola turn right to walk slowly (!) with a bunch of zombies behind killing a few every second.
The door to your left (back to the teleporter) stays closed all time.
Just open the door and head straight towards the stairs always killing zombies.
Go over the bridge and you're back on square one.
IF there are too many zombies behind you just jump down the ledge again, but this time
walk LEFT through the gap into the "Thompson Room" and activate the trap.
Wait until round is over and continue with this method.

For the doggie rounds:
Hide your ass up where the cage (box spot) is.
But don't all the way behind, stay on the window you can jump down.
Let the dog come close and one hit kill him with a shotgun shot.

Have your weapons punched for round 13 to make the dog rounds very easy.