Not as tough as the others, believe it or not. once you buy the ultimate Jewel pick a character that has an ultimate move like Goku for example, then fight Kid buu. The fastest way to get the Energy bar up is to MAX the chases. Once you're ready make sure Kid Buu gets beat down enough that he teleports behind the MOUNTAIN, that way he can't hurt you when you do the ultimate charge up.
I did this with Goku and 3 Majin buus, their healing is incredible and their health is huge.