At the beggining dont buy any guns, untill you run out of ammo.
For the first 2 levels just stab that zombies as they die in 1 or 2 knife melees. At level 4 buy upstairs then if u have enough buy the other set of stairs down 2 the help room, DO NOT OPEN THE HELP DOOR!!!!
One player waits by the shelves at the bottom of the stairs while another 1 runs around in the help room covering all windows, if you are playing with 4 players do the same but 1 window 4 each player (other then 1 that needs 2 cover the wall) u should get up 2 at least level 20 if done correctly.