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 (if you eventually get a ballistic knife buy a bowie knife as soon as possible zombies die fast till round 20), and then start to use your pistol and grenades.
Once round four 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 buy it!!!!!!!! No other perks are needed except maybe speed cola and quick revive. Try to get a good gun such as the ray gun, rpk, 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.
(Quick Tips From Skull): In the beginning rounds starting at 2, you should shoot your pistol 5 times into a zombie's torso, or 8 times in their leg, to get maximum points in those low level stages. Once the acquired amount of shots are taken, knife and kill. Once zombie kill with 5 shots and a knife will score you 180 points, with 8 shots you'd get 210. This is a lot more effective for getting the maximum amount of points in those low levels.
Also, keep your pistol for as long as you can. You can easily munch up the points by using it on zombies that take about 15 shots to die. However, only do this if it's at the very beginning of the round or if all zombies have spawned and you are leading them on a mad chase throughout the map. The same method can be used with machine guns, lead them on a chase, use the stakeout to kill the ones in front of you that get in your way, and once all have spawned and they're behind you, turn and run at the same time (obviously) and fire from the hip. You'll score major points. I got to level 26 on solo using this method.
Hope it helps!