Scored a new best 2 nights ago, level 11 and 190 kills. Key is to score potato it on the early levels to get your points up. First use nothing but your pistol on level 1, let some of them break the barriers so you get points for rebuilding. I stay in the first room as long as I can, usually to about level 4 or so. Then go through the door with help written on it. DO NOT move the couch at the top of the stairs in the first room. Stay in the back room as long as you can. Another helpful hint is at the end of each round to leave 1 zombie alive preferably one with his legs blown off so you can run around and rebuild barriers and get weapons before you kill him and move to the next round. The other key is the random weapon box, it's through the help door on the left, looks like a coffin with ? marks on the top. Try and save up as much loot as you can before you start getting weapons from it. Each time you use it it's 950 and sometimes you will get a junk weapon and not want to take it. The best weapons in that box are MG42, Browning 1919, FG42, and the RAY GUN. The ray gun is friggin glorious, but it has splash damage so you can't fire it at zombies that are too close or you will kill yourself. Imo the best weapons combo is either MG42/Ray, or MG42/Browning. Hold out in the back room as long as you can then move the couch and go upstairs. In the upstairs run to the back corner where you can get the "stielhandgrenades", next to the couch you never moved from the first room, this is the best spot to make your stand. From here the zombies can only come from two directions, through the door to your front, and the window to your right. They really start coming fast on the higher levels so chuck lots of grenades through the door, and just keep an eye on the window to the right. Make sure to be reloading on pauses and try and get the max ammo boxes. At the beginning of rounds you can get some by throwing grenades outside, and down onto the ground floor underneath you.
EDIT: I only ever play it single player so that is my strategy for single player. I'm sure it works with 2 people just as well since you can cover each other while you reload and such.
