This can be gained pretty easily just by having Cartman in your party, but if you don't want to keep him in your party to get this, there is a very easy way to farm this. Go to the playground and go to the right side to find a ship, at the bottom of the stairs will be some kindergarten pirates. Attack them and then use Cartmen's Burning Cloud ability twice to kill them (just have your character attack but press no buttons, you'll do no damage). If you want to make this go faster, have your main character use a speed potion on Cartman so he can go twice in a row and wipe them out before they can do anything.
From here there is an area straight south, go to it and then go right to a new area (has railroad tracks). Now return to the ship to find they have respawned. There's 3 of them so you'll need to do this 7 times, but it's very quick, especially if you use speed potions. I don't know if it's even possible to get Cartman at a low enough level to not have 20 PP, but you'll need 20 PP to use Burning Cloud twice.
As an aside, I guess you could use this to level up but the level cap is 15 and easily reached if you are trying to do everything, so I wouldn't bother. This was done during day 3, shortly after getting Cartman as a buddy.