Open the room that says "Help" during the third or fourth round. It will supply weapons that will be useful in a crucial battle. Note: There are four openings that the zombies will try to break through. Never open up the stairs; it will result in more zombies.
Start any sort of Nazi Zombie game and unlock the door to the Mystery Box room, then g to the Mystery Box. Go over to the right and look at a table to find a radio. Shoot a bullet at the radio then wait a few seconds. You will hear random music such as rock, disco, opera, and some in game music such as the Russian victory song, the last level music, or the Japanese combat music. The music will stop in between rounds but will then start again. Every player in game can even hear it.
At the end of every round, do not kill the last zombie. Kill all of the others, but keep one live. When it has entered the building, toss a grenade at it to blow off its legs. It should now crawl slowly on the ground, leaving you plenty of time to patch up any windows you need. The next round will not begin until you kill him. He is easy to evade. Use this opportunity to get ready for the next round. When you are ready, kill him and the round will end.
This works best in online multi-player mode. After the first few waves, open the upstairs and not the door. After that, open the other stairs leading into the gun room. You will only need to worry about that room and the stairs. Have two people guard the stairs. The room has less obstructions to get caught up on and more space to move. If overwhelmed you can open the door, but the next round will be your last.
There are two methods of doing this trick. In the upstairs over the first starting room is a room with one window and a doorway with a window view. Use that door way to bottleneck zombies while having someone else guard the stairwell and window. You can either unlock the door then the far stairs. This will keep your back free. The other way is to unlock the first stairs, leaving no protective sides. The only downside is eventually you will need ammo. This works well online, as long as you work together.
Get to level three and leave a Crawler. Open the door with steps (not the ramp). Then, open the door that leads to the Thompson and then clear the debris. Jump down and turn the power on. Nown run out and make a right. Keep going straight. You will pass two windows on your right and a door on your left and right. Do not open the doors. From this area, take out the zombies that come down the path and have someone watch the window in the back. For the dogs, go to mainframe and hide by the fence that is in the corner by the window. When the dogs are cleared, return to the path and continue doing this. Do not forget to leave a Crawler or a zombie in the window and be careful getting the nukes so you do not kill the final zombie so that you can get perks or whatever else.
For the first couple rounds, let all of the zombies break down the boards and come inside. Then, use your pistol or knife to kill them to earn more points than by simply shooting them once in the head. A knife to the head will earn 130 points per kill. Once all of the zombies are dead, patch up the boards. You will have much more money than if you simply killed them as they approached. Note: After level 3, the zombies are too powerful to be taken down this way.
When you have over 950 points, go to the door marked "HELP". There is a box on the far left that you can use to purchase a random weapon from.