The first thing to do, is to establish a plan. Depending on where you are, you may or may not have an easy time beating Yunalesca. For the most part, I will assume Auron can only do 1750 on average. If you have Holy already, then you need mental help. Seriously. Excessive level is for excessive idiots.
The first stage of the Yunalesca fight is straightforward. Any action you take against Yunalesca will be countered with an attack that causes an ailment that will hinder such an ability. If you attack physically, you will be Darkness-ed, a spell will incur Silence, and any other action will be countered with Sleep. The best way to finish Yunalesca is using Tidus, Auron, and Wakka (or Kimahri is his strength is high enough). Use Aim when she Darkness-es you, and you should be able to fight her effectively. Bring in Yuna to heal as needed (Yunalesca will not Silence her if all she does is Cure). Haste/Hastega as needed. Don't bother with inflicting an ailment on Yunalesca, she is immune to them. Save your Aeons for later in the fight.
Get the characters who you plan on not using to get at least one action in.
I got through the fight with Tidus, Auron, Wakka, and Yuna.
The second portion, Yunalesca stops using Darkness, Silence, and Sleep, so heal the conditions. Instead, she will counter every action against her with Hellbiter, an attack that hits all of your characters, and causes the Zombie status. For the most part, this is currently detrimental to your party survivng, so use Holy Waters (not Remedy), and Heal. However, once her HP gets low enough, do not heal the Zombie status. She also occasionally casts Regen and Cura on your characters, so you can remove the status and benefit. For now.
In the third portion, she uses Mega Death which will kill any un-Zombied members instantly. If you are Zombied, you will survive the attack. Unfortunately, you will suffer from the attacks that were once helping you.
From here, you can go multiple ways to fight the Third form. You can get Yuna to unleash all of her Aeons' Overdrives (Yuna is unlikely to die). If you DON'T use Aeons, then prepare for lots of reviving and Curing. To counter Regen or Curaga, you can set up Reflect spells, Shell spells, or wait for Yunalesca to cast Regen, and Dispel it.
If your characters die during the fight, revive them and Cure, and hope Yunalesca uses her turn to cast Hellbiter. This should keep your HP high enough to survive a few more rounds, as her only major form of attack is Mega Death, Hellbiter, and a bolt of what seems to be something like ice (but is non-elemental), that dispels all positive ailments on your characters.