Firstly, make sure your party is at high levels with Limit Breaks. Train A LOT. And also have some decent equipment, and some Holy Water. Make sure you guys have Hastega so you can cure the person that gets hit by Lance of Atrophy. Next, have Rikku with her overdrive so se can use Super Mighty Guard. To get that spell, beat Belgemine until you get 10 chocobo feathers, then combine two of them, so that you can survive Total Annihalation. Make sure Yuna has some good curing spells, so whenever Seymour takes out Cross Cleave you can heal, 'cause Total Annihalation will come up after that soon.

Still, the best thing you can do is TRAIN TRAIN TRAIN! :oscar: