Another way to do this without having to worry about any zombies:
Go to the 'cowboy' styled restaurant on Food Court. They have a small bar with drinks.
The catch is: It seems like that bar has so much booze even alcoholics will say at one time: Stop it! Keep grabbing the bottles out of the rack, after grabbing some bottles you get 1000PP (peepee, haha!) per second. Takes about 2 minutes to gain a level!
Obviously best done with a turbo controller, with co-op I bet it will be much faster and prevent RSI.
(Also: You have to be around, since at some levels you will receive a Combo Card. (Those cards giving twice the PP showing you how to build stuff)