Ending The Dream
When Yuna decides to find a permanent way to defeat Sin without sacrificing one of her Guards, Yunalesca stops her party from going any further.
She doesn't want Sin to be truly defeated, nor does she want the truth about Spira's light of hope to be extinguished, so she tries to kill them all.
The inevitable battle against the three forms of Zanarkand's princess is arguably the most difficult story battle in the game.
If Rikku hasn't finished her area of the Sphere board, you're likely behind on levels. Get some Full Zombie Defense equipment by modifying your best equipment with slots into using Holy Waters (bought at Gagazet Mountain). Also try getting Full Death Defense equipment, along with Auto Status Recovery types. Then stock up on Panaceas, Holy Waters, and Eye Drops before charging all your Summon and character Overdrive meters to full.
If Rikku hasn't finished her area of the Sphere board, you're likely behind on levels. Get some Full Zombie Defense equipment by modifying your best equipment with slots into using Holy Waters (bought at Gagazet Mountain). Also try getting Full Death Defense equipment, along with Auto Status Recovery types. Then stock up on Panaceas, Holy Waters, and Eye Drops before charging all your Summon and character Overdrive meters to full.
Yunalesca: First Form
Hit Points: 20000 (overkill 30000)Magic Points: 500
Physical Strength: 20
Physical Defense: 50
Magic Strength: 30
Magic Defense: 40
Agility: 40
Luck: 20
Ability: Absorb
Elements: Weak to Holy
Preparations
Equip Complete Darkness Defense protectors on Physical Attackers (ex: Auron, Tidus, Kimahri), Complete Sleepel Defense protectors on your Thieves/Status attackers (ex: Wakka, Rikku), and Complete Silence Defense protectors your Magical Attackers (ex: Lulu). If you don't have Complete Defense abilities on equipments, the other ones have a 50% chance of protecting you. A combo of Darkness and Sleepel Defense is best for Wakka and Rikku.
The Battle
Yunalesca's normal attack does a little damage and negates any positive statuses on your characters. Her main attack in this battle is Absorb, which drains about 1/2 of your full HP. If you use a Physical Attack or Mug, Yunalesca will counterattack with Blind (effect lasts 3 rounds). If you use a Magical Attack, Yunalesca will counterattack with Silence (effect lasts 3 rounds). If you try Stealing or a Status Attack, Yunalesca will counter attack with Sleepel (effect lasts 3 rounds).
Don't use your Summons yet, as Yunalesca will kill them off after one attack. Don't bother using Haste, Shell or Protect spells as Yunalesca will negate them. She is also protected from any Status attacks so don't use them.
Being the first, this is the easiest form of the three. This is the best time for you to use Rikku and Steal some items. In my case I used a Physical attacker to pummel her and have Rikku Steal/Mug while keeping everyone's HP above 1/2 of their maximum at all times. If anyone needed a Cure spell or Esuna, I just used Yuna or Rikku's Al Bhed Recovery medecine, if not the appropriate items.
Yunalesca: Second Form
Hit Points: 48000 (overkill 58000)Magic Points: 500
Physical Strength: 20
Physical Defense: 50
Magic Strength: 30
Magic Defense: 40
Agility: 40
Luck: 20
Ability: Absorb, Hellbiter
Elements: Weak to Holy
Preparations
Now's the time to equip the Full Zombie Defense protectors. Have least have one person with full protection, or it will be a difficult battle, because the only way to cure Zombie is with Holy Water items. The other option is to wait and revive someone. Try to also have someone with the See Through (like an Auto Libra/Scan) ability on a weapon so you can check on Yunalesca's HP from time to time.
The Battle
At the beginning of the round she does a new attack, "Hellbiter", which not only does damage but it has 100% chance of casting Zombie on the whole party. To add to the new attack, she uses Cura, Cureaga and Regen on your characters afterwards. Being in Zombie status means that those spells will do damage rather than heal. She will continue to use all her abilities from the previous form.
If you have prepared the right defenses, these spells will heal you as they normally should and you can pummel away with your Physical or Magical Attackers. If not, use a fast character like Rikku, Yuna or Tidus to recover party members with Holy Waters or tag Zombie members in and out of the party. If you're lucky, Yunalesca may cast the recovery spells on non-Zombie characters.
There is an ideal time for using a charged Summon Overdrive, and that's only when you are sure Yunalesca will be KOed right after the hit. This is because right after the second form is KOed, Yunalesca will do an attack called Over Death on the whole party and it would be game over. Over Death causes Instant Death on any non-Zombied character. You really need to have a character with Full Death Defense in the active battle party or better yet, use a Summon since Summons are not affected by Status attacks. At this point in the game most of the regular Summon Overdrives do about 9999, so keep Yunalesca's HP in mind.
You have to prepare beforehand, making use of the Change Equipment option. After every few rounds of attacking make sure to check on Yunalesca's HP. After all that trouble, it can be frustrating to have a Game Over. I suggest using Valefor's Summon to kill the second form. Valefor is the weakest and it's best to save the more powerful Summons for the third form.
Yunalesca: Third Form
Hit Points: 60000 (overkill 70000)Magic Points: 500
Elements: Weak to Holy
Steal: Stamina Tablet, Farplane Wind
New Ability: Over Death, Aspira, Mind Blast
Sphere Drop: Level 3 Key Sphere
Weapon Drop: Piercing, Zombie Attack
Armor Drop: Full Zombie Defense
AP: 14000 (overkill 21,000)
GIL: 9000
Preparations
Now that you've survived her Over Death attack, put on your Full Zombie Defense protectors and Full Confuse Defense protectors on your main characters. Have least have one or two people with the best protection, or it will be a difficult battle.
The Battle
Yunalesca will start the round with Over Death, which causes instant Death on non-Zombied characters, so try to have at least one character Zombied at all times. Now she adds yet another Medusa-themed attack, Mind Blast, which causes Confuse on the whole party and does damage. Finally, on top of that she also uses Aspira which can drain all of your MP. She will continue to use all her abilities from the previous forms, though Absorb will only be done to Aeons.
If you have prepared the right defenses, Cure spells will heal you as they normally should and you can pummel away with your Physical or Magical Attackers. If not, use a fast character like Rikku, Yuna or Tidus to recover party members with Holy Waters or tag Zombie members in and out of the party.
Now is the time to waste the rest of your Summons' Overdrives and pummel her with your strongest Physical Attackers and Magical Attackers. If you don't have Death defense equipment and none of your characters are in Zombie status, you're at high risk to fall victim to Over Death and have a Game Over.
If by some chance your Summon lasts longer than one round, don't bother with wasting a round to heal them, simply attack as much as you can with the most powerful attack until the Summon is KOed.
This probably the most hectic battle if you have no Summon overdrives to shed off all that HP she has, Bahamut's Overdrive is the best choice for the killing blow if you want an Overkill. Having characters Confused, Zombied and no protection is the worst situation, much less the instant game over when she does Over Death. So always modify your equipment beforehand.