Yunalesca is pretty tough but I sorta beat her like this. One thing for sure is 'DON'T USE AEONS.'It's very annoying. You should train up for full overdrives to add to the attack. Her Hellbiter is quite annoying but if you have a few Holy Waters, they should help. On the third round she uses Mega Death, where anyone not a zombie dies instantly. This is tricky because if you had armour equipped protecting you from Zombie, it was pretty much a waste. I used Yuna's Holy attack most of the time and used a Mana Tablet on Yuna for a bit of extra firepower. Her Mind Blast is annoying but can be countered by attackin the confused or having Confuseproof armour or use Esuna. Another method is to give Yuna all the best stuff and save her till last. Have the lads with the best attack out front and give all you got before releasing Yuna at the end