For the first 2 rounds, just knife the zombies. (I do it up to round 4.) Then, once you have enough points, open up upstairs. Stay up there to about round 5. Then, buy downstairs, and stay in the help room (DO NOT OPEN HELP DOOR!) until you get the raygun, and the flamethrower. Once you have those 2 weapons, go back to the room you start in, and go over and camp by the help door. Just keep on using the raygun and the flamethrower and the raygun, while running around that one pillar by the help door, but remember, NEVER OPEN THE HELP DOOR! or else it ruins your strategy. This may work online too, but it's mostly for solo. Good Luck!