From the official Brady Games FFX strategy guide:
Before proceeding into this fight, you need to equip your part members with armor that will protect against status effects. In particular, status effects like Death, Zombie, Darkness, Silence and Sleep.
Yunalesca's first form is easy to defeat, so don't use any overdrives yet. Yunalesca responds to each physical attack by inflicting Darkness, and each spell ast on her is met with Silence. She also responds to Special techniqus, such as Steal with Sleep.
In her second form, Yunalesca unleashes a new attack, Hellbiter. This attack inflicts zombie status on your characters, so quickly use a Holy Water or Remedy. Yunalesca will then cast healing magic on characters to damage them, so if you remove the zombie effect quickly enough, you can benefit rom this. Make sure that Haste is in effect on the entire party.
It's important to keep a balance between having characters with zombie effect while some remain normal. Another way to handle the status effects is to switch out party members whenever the need arises.
When the true face of Yunalesca is revealed, cast Haste and Protect on the entire part. Also, do not remove the zombie effect from the paty. any character who is not afflicted with zombie will be killed instatnly when Yunalesca performs her Mega-Death attack.
If a character is hit with regen, use Yuna's Dispel to remove the constant healing effect without negating the Zombie effect. Also, bring some of your strongest aeons and perform their overdrives.
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Okay, hope that helps. My main party for this battle was Auron, Tidus and Yuna, as usual. Auron needs to be safe against Darkness, so he can inflict mass damage. Yuna, healing as usual, whenever needed. Tidus (can be substituted with Wakka or Kimarhi I guess) should be your secondary attacker, and item-user. Character overdrives should be used on the second form, aeons on the third. If, for some reason, you still have your aeon after the overdrive, you can use them as sacraficial lambs.
Auron's overdrive should be set to Warrior/Soldier (one that goes up when you do damage). Yuna's should be set to Comrade. That's all I can think of. Good luck.