OD all your aeons and characters on Mt Gagazet (or is it in Zanarkand?)
and you'll be able to take her third form down in about 10 seconds ;\

I always kept 3 people in zombie...if 1 got real low HP, you can let him die
and just Revive him or use a pheonix down, or you can holy water him and
heal him back up. In any case, the key is to get the guy with the highest
defenses and the highest HP in zombie and keep him there ;]

It took me like 8 battles to beat Yunalesca so don't get discouraged. After
7 times I decided to overdrive all my aeons and characters, I only had to
use like 2 aeons and Tidus to kill her third form ;]

.opt