Use your knife on the first couple of rounds. It will give you more points than killing with a weapon. That way you can open new doors, and get better weapons. Speaking of that, pump action shotgun! Also, when you start opening new areas, you have to remember to circle back and close up the boards. If you forget you'll have super waves. Best way to do that is to leave a slow walker, and just run from him. This is easier after you have opened up all of the areas. Until you kill him, you don't have to move to the next level. Just be careful, they are freaking sneaky. You can also shoot and repair at the same time. It just takes a little finess.