Start the game and forget about Katie. Go unlock the weapon shop and grab two shotguns and put on the moosehead. Using the moosehead, run to the hadrware store and grab two pitchforks. Combine them at the workbench next door and put the moosehead back on and run back to the gas station and go inside (this'll respawn the shotguns, pitchfork and moosehead). Come right back out and, with the moosehead still on, run back to the gun shop and grab two more shotguns (your inventory is full now). Drop your moosehead and grab a new one and run back to the hardware store. Drop the moosehead inside (it's eaiser to pick it up inside if zombies surround you) and grab one pitch fork. Run next door to the workbench and combine the pitchfolk with a shotgun and then leave a shotgun on the bench. Go back into the hardware store and get one more pitch folk and go back and combine it.
You should have four pitchfork shotguns now. Put your moosehead back on and run back to the gas station; but instead of going inside, go to where the zombies gather in front of the pumps. Get a nice crowd around you (surrounded on all sides is the best) and then start randomly sticking into the crowd with the strong attack (the one where you hold down X and lift the zombie up and shoots it three times). The one zombie you kill with give 2500 points, and sometimes you'll pick up 200 bonus PP for nearby zombies caught in the spray. With four pitchfork guns, you'll net over 100,000 PP in about 2 hrs of game time (2500 PP attack x 10 shots (since the gun holds 30 ammo, but the strong attack uses 3 shots) x 4 guns = 100,000 PP). When all four pitchfork shotguns are used up, run into the gas station and back out to respawn everything and repeat.
Figured this out messing around in a playthrough and realized how much damage the strong attack does so I decided to do a playthrough just repeating this. The moose head is used to get from point A to B quicker (and I try not to kill too many zombies with it so it'll last). Other places might work better than the around the as station pumps, but I found it worked for me because, if I messed up, I could duck into the weapon shop for a sword and whiskey.