seriously the best things to do are
1. hop up on her back and just thunder the crap outta her ( make sure that goofy knows MP gift, turn his magic consuming things down in the costomization screen and turn up support actions ) eventually she will be low enough that you can just hack away at her head. OR
2. their is a giant root behind her ( you can see it when she changes, it looks alittle bit like a pipe ) you can jump up their with glide or something like that and just stand there while goofy and donald beat her up, and don't worry, if you in the right spots, NOTHING she throws at you can hit you