For this strategy you will need four people.

For the first four rounds you need to stay in the lobby. Knife (hint: whatever round it is that's how many knifes it takes) until the fourth round, then open the door upstairs.

Buy the PM63 (trust me this is great for the beginnig of the game) and wait until after the dogs to open the next door.

Go down the right set of stairs and swith to your pistol then buy the MP40. Have two people go back up the stairs into the room next to the PM63 (the one where the zombies drop from the hole in the ceiling) while one person gaurds the two downstairs and one upstairs (next to the stakeout).

At round ten open up both doors and go into the power room. Once you end round eleven turn the power on and link the teleporter.

After that find out where the mystery box is and go get it, then have everyone buy juggernog, then start a circle... If you dont know what I mean, I mean keep travelling through the doors that you have already opened. If someone gets down don't revive them unless there aren't many zombies or unless the other two people are willing to defend you. If not go in the circle and if you get to them in time revive them - if not oh well.

The balistic knife would be a big advantage, if you get it in the mystery box early, hold on to it then Pack-A-Punch it later, also try to get the bowie knife because after you upgrade the balistic knife, not only you will be able to revive downed allies but its a one knife kill through round thriteen, a two knife kill until round 29, and so on...