This strategy only really works for 2, 3 or 4 man zombies. And me and my friend got to 29 doing this strategy.

First of all, open the right side to the box then turn the power on. Camp in that courtyard until you get decent guns, preferably an mg42 and a ray gun, or ray gun and trench gun (for covering windows). It would also help if atleast one of you had a monkey bomb. When you have good enough guns, start activating all of the teleporters. When they're all linked, buy Juggernog and Speed Cola (you can also buy quick revive if you have enough).

When you have good weapons, perks and have linked the teleporters, go through the left door in the first courtyard, the other door you first opened, and then go in the doorway opposite that window (the corridor with an electric fence). Sit right at the back of that corridor, and DON'T clear the debris on the stairs, otherwise the strategy can't work.

If you are doing it 2 man, have one person (with ray gun and trench gun), cover the two windows opposite each other. The second person (with mg42/ppsh and ray gun) should cover the corridor.
If you are doing it 3 man, have one person on the two windows once again, and have the two others cover the corridor with the machine guns and ray gun.
If you are doing it 4 man, have 1 person on each window, then the remaining two should cover the corridor. If the two covering the corridor get overrun, one person on a window should go and help out. The remaining one on the window should now cover both, until everything is back under control.

Once you all have over 10,000, try and keep a zombie. If you don't have a zombie left over, then play the next round and try again. When you have a last zombie, have one person look after him, the others should then go and upgrade their weapons, and buy any perks that they might have lost. Once everyone has upgraded both their guns and got Juggernog and Speed Cola, go back to the strategy corridor, and continue killing.

• It is recommended that all of you have an upgraded ray gun.
• It is recommended that atleast one of you has a monkey bomb.
• It is recommended that you buy betties, and place them at the back.

If you get too overrun, you should clear the debris, and jump down into the courtyard where the box starts. Run towards the teleporter in the room near the bowie knife and quick revive. Once you are all in the teleporter, teleport back to the main frame, and just keep running to different teleporters and turning on electric fences.

For dogs, you should all get right on the back wall. Get your ray guns equipped, and shoot the dogs. They should only come from in front of you, unless you are too far forward.

I hope this strategy helps you get to a high round!!