The lights are on
All right, if you follow this guide exactly, you should be able to make it beyond level 10 easily and accumulate and absurd amount of points in the early rounds. For rounds 1-4, stay in the foyer. Knife the zombies for the 1st two rounds, and then start to use your pistol and grenades. Once round for is over, or if you get swarmed during it, dash to the top left and keep going until you reach the MP40 room with the speed cola. Everyone buys an MP40 and two people go up the stairs to prevent the zombies from going down to the level with the stakeout. If you get swarmed at the stairs, call for help or back down. After round 8, buy all the way to the stage. One person mans the back entrance, and three take the dressing room entrance. If things go haywire, turn on the power and begin to cycle the map. However, the second you can, buy Juggernaut. No other perks are needed except maybe speed cola. Try to get a good gun such as the ray gun, commando, or thunder gun from the box. At the beginning of every round after the power is on, stay in the teleporter and warp once you get swamped. Throw grenades from above. When you return, cycle the map and shoot periodically.
Hope it helps!