Before going into this battle, equip everyone with armor that will protect against status effects. In particular, Death, Zombie, and sleep. Also, for Auron, Wakka, and Tidus equip armor that is resistant to Darkness. Likewise, equip Lulu and Yuna with armor resistant to Silence. Equip Kimarhi and Rikku with things like this according to there attack and magic.
Don't use overdrives on Yunalesca's first form unless you really have to. She uses Darkness to counter any physical attack, and any magic used on her is countered with Silence. She also responds to specials like steal, with Sleep.
In her second form, she uses Hellbiter so make sure you use a Holy Water or Remedy on everyone that is Zombiefied.(Hellbiter causes Zombie) She then heals anyone that is Zombied.(this kills them bigtime) Ohh, and remember to cast Haste or Hastaga
Next, is the hard part. When she rears her trully ugly head(third form) cast haste and Protect on the fighters. Also, you will need to have someone who is a Zombie on you side or you will die immediatly when she uses MEGADEATH. Zombies are uneffected but everything else dies. (Also, use Dispel on any Zombies that have Regen cast on them) Don't forget to use all of your Aeons' overdrives on the third Yunalesca.
Good luck! (you won't need luck with these tips)
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