Paper Mario: The Thousand-Year Door Tips

Charge a lot while fighting bosses
While fighting bosses, the easiest way to defeat them is to Charge a few times then Power Bounce.

An even faster way of doing this is to upgrade Goombella to Ultra Rank so that she knows how to use Rally Wink. Charge with Mario and use Rally Wink with Goombella so that Mario can charge again. This makes charging up attack power require half the amount of turns.