Paper Mario: The Thousand-Year Door Tips

Super Easy EXP in Keelhaul Key
After you beat chapter 4, you go to the Island, Keelhaul Key. When you start to explore, go fight a green fuzzy. Tattle with Goombella if you haven't already, and then attack some of them. When there are one or two left, DON'T KILL THEM.

Let them multiply so you can get 3 EXP for each one you destroy, and if you let them keep multiplying, they'll add up to a lot of EXP. I got 33 just from one fight!

But this will only work if you're just at chapter 5, because if you're too strong, they won't give any EXP and you only get one for the whole battle.