I use my own stratergy using 4 people but I'm sure many others may do this to. First we open the door upstairs and go straight to the stage, then its one at each window and two at door by the dressing room, but don't turn power on (you need Claymores).
At round 8-10 you turn the power on, if you found the box at early round and have decent guns, leave at 9, if one has Ray Gun/Thundergun, leave at 8, and you get the idea.
Now you open ALL the doors and stay in pairs! First you all start at the teleporter, one with ZeusCannon (Thundergun) and Ray Gun, then everyone else with Ray Guns and if you can get Thundergun more than once, then all have Thundergun.
Plant Claymores around teleporter and as soon as zombies reach you, teleport.
Throw frags to kill as many zombies as you can.
Now you are back in the lobby whoever has the ZeusCannon should run straight for the Trap and activate it. Now this is where you part in pairs, one pair goes upstairs and one downstairs, though I recommend whoever has the ZeusCannon should go downstairs since you are more likely to get trapped. As you go through each room camp in there for atleast 10-15 seconds, when you leave turn the Trap on and it takes alot of zombies off your shoulders (though keep checking for Max Ammos, Nukes, Carpenters and Insta-Kills).
Finally you should all meet up at the stage, if there are still alot of zombies, turn the turret on, then run straight through the theater and part paths once again.
Some people may think this may not work, but trust me, it does! Its similar to what alot used to do on Der Riese and alot used to get to levels 35+ (going in teleporter then working your way back).
NOTE: You can do this as just a pair too, though this is much more dangerous.
You can also do this on your own, though I would highly recommend buying Revives and Juggernaught!