The Dark Koopinators helped me out a lot. I would just Quake Hammer over and over again, keeping them immobilized while I refilled my star power with appeals and my HP and FP with Happy Hearts and Happy Flowers.
Spoiler...
Bonetail totally caught me off guard. I thought once I got to level 100 it would be over with. I had no items left, and the only partner I had left alive was Koops. The battle actually wasn't that hard, but it was really long. Bonetail kept healing himself, and he could never hit me (Lucky Day + 4 Pretty Lucky).
