Just a quick tip for leveling quickly..
First of all, build up some cash via the method here
make sure you grab the combat magazines.
Go to the pawnshop on SilverStrip and buy as many Tesla Balls as you can ($35,000 each)
Start the "extreme off road" challenge. Grab a ball and throw it. You will get 275pp per kill with the Tesla ball until you level up enough to get the combo card where you will hit over 400. Each ball will kill between 10 and 20 zombies.
I started doing this at level 36, and hit level 45 in about 45 minutes, meaning I only have to grind out the remaining 5 levels. If you alternate it with the six digit?!? achievement method above you can top up your cash and kills + hitting the 20,000 pp bonuses.
*update* Don't forget the horror magazines for even more bonus points - your 500+ kills will net you 750pp and 30,000pp for 1000 kills.