I'm at 80 for the VI and I need to beat Chac, well she's kicking my butt. I have everyone using the Supreme Light GG but I can't get auto-life to work. But the moment I start she just does Heaven's Cateract and we all die.

Currently I have Yuna as a Gunner, Rikku-Alchemist, Paine-Dark Knight and they are all lvl 99. Yuna has catnip but i can't keep everyone alive long enough to utilize it! Help!

One trick is to have Chac oversouled. Head to Mushroom rock road and keep fighting the basilisk types there until one of them oversouls. Run away and go straight to the VI. Oversouled Chac won't use heavens cataract.

hmm well I did figure out how to use auto life but i also figured out that it only works once...that sucked. I'm taking a break!

You might want to try the guide on gamefaqs for the VI for tips. Another option - have someone keep using the delay attack (can't recall the class). It will work on Chac and stop her from acting as often.