Before you start equip weapons with Confuse proof, if you dont have them it may be a pain so Steal Musks off those 1 eyed Flying things in Gagazet.
Ur first lineup should be Tidus, Yuna & Auron/Wakka, Cast reflect on everyone with Yuny to Reflect her status attacks & Shell on Yunalesca for later & heal with items when needed, use Yuny, Tidus should Hastaga and then hack away with him & Auron/Wakka.
In her second form she will Zombify everyone with Hellbiter, then cast Curaga on you, this isnt a problem as you have reflect cast & shelled her, if she casts Regen, Dispell it immediately, dont be tempted to cure Zombie status & she'll cast Hellbiter again & you will waste a turn, when she punches she will disable any Protective Spells (Bar Reflect) on you so dont bother wastying time on that, when she kills off a character, revive immediately. Be sure keep at least 1 person Zombified for round three DO NOT BE TEMPTED TO CURE IT! Why?
Round 3 Mega-Death=Everyone Dies...Except Zombies & People with Deathproof armour. Hack Hack Revive Hack is the key once you Cast reflect, this outta do it, max Wakka or Auron to do at least 5000 damage a turn & Tidus 3000. If you have Holy, cast it, if not swap Yuna with Wakka/Auron to share in the hacking, I founbd her easier than Seymour Mortis actually...
Good Luck!