You can use this alone (but the more the merrier!).

1. Don't open the stairs or the help room, that just makes more windows for them to get in.

2. If a zombie is in the building shot it 4 times with a pistol, then knife it and you will get tons of points.

3. If you must open the stair or help room them always check the windows and repair because it will make it harder to get in and you get 10 points.

4. Take advantage of insta-kill, if they are in a huge group throw a grenade and they will all die, or if they aren't knife them for tons of points.

5. Don't waste your points on crap! Get good guns like the shotguns and try to get rifles (damage counts more than rate of fire).