umm.. i beat her on my first try, didn't have Holy at that time. No member was KO'ed (i must be lucky )

basically, i kept lulu, yuna, auron, and tidus for the main party. Yunalesca's first form, i used Lulu since she has silenceproof on. Then, 3rd and 4th from, i had yuna, tidus and Auron. I used 3 Aeons to do the bidding for me But when yunalesca hits with Hellbiter, i have yuna give holywater to tidus because he's faster and have tidus give holy water to auron so auron can attack and won't get hurt in anyway (he has the most HP, i want him to live ), It just went on like that and then when she has few HP left before being destroyed and do the mega death, i made sure i stall until it's her turn so she can use Hellbiter to turn all the characters into zombie and then auron unleashes the last strike and 3rd form comes.

I used Aeons to fight the 3rd form since the aeons needed some training and it's a safe way to prevent the party from dying. After aeons all die, i used auron and tidus to attack while yuna heals and cures zombie.