If you are back at your base early waiting for the helicopter there's a nice easy way to level up. Make sure to have some guns with you, make your way up to the top of the warehouse and you can shoot down all the federal agents patrolling below without them shooting you. Each guy is worth 5000 PP. Whenever you run out of ammo (or guys to shoot), just jump down and grab some of the machine guns they leave behind, take the elevator back up and do it again to a fresh batch of unsuspecting AI.