Ok she is very hard for two reasons. One the controls for the level are somewhat akward and difficult to control, and two she is a hard boss plain and simple. I would do a few things.
One: equip Goofy and Donald with your best weapons. (Give Donald power over magic, and same for Goofy.) Sora needs to have a Keyblade with extra MP added, and/or magic power increased.
Two: equip your faithful hands with the strongest accessories primarily in defense and offense. But keep Sora strong. Try to give Donald MP, magic, and offense. Give Goofy AP, offense, and defense. Use the second chance ability, MP rage, and rage abilities on all characters if available.
Three: get used to the level, practice swimming around and get the feel for the level. You will encounter the same configuration at Neverland.
Four: fight and fight hard. She is not easy but can be defeated. When you start get a lock on, then use your beefed up partners to whoop up on her. Then use magic and items to keep them alive. She is weak to thunder magic so use it. And just keep pluging away, you can and will beat her.