I'm guessing this has already been mentioned but by jumping and pressing the "sleep" button while you're in the air your character will freeze in the air, and upon waking up will be able to walk and shoot in the air. By doing this multiple times, you can get your character above the screen and can simply shoot the enemies without them being able to reach you.
The AK-47 is in the smallest town, can't remember which building though. I won with only one survivor by giving him the AK 47 and just using the chainsaw against anybody at the wall.