she is incredibly hard is you don't have a plan. But i'm going to keep this simple. These are some tips. I beat her on my first try but she is a pain.

1. bahamut is your best friend . Finish her off with him. Make sure either he has overdrive or yuna does.
2. When you reach her third form, keep your guys in zombie form. At least two guys should be in zombie. When she unleashes Mega-Death, you won't all be annhilieated. Trust me it works
3. If you have holy, use it. It really helps.
4. Put you're two strongest guys in (i used tidus and auron) and yuna
5. Have tidus cast hastega when needed. Otherwise let auron and tidus hack at her and have yuna heal.