Instructions
Level your party to at least 60. Ixion will be nearly impossible to defeat for any characters below this level.
Apply Reflectga before the battle begins. This will protect you from Ixion's initial blast of Silencga, Sleepga, and Curse.
Assign one person in your party the primary role of dispelling and cleansing Ixion's various status attacks.
Equip a Rubber Suit when Ixion begins to charge Thundaga so you can reflect it back into him. This powerful attack can level your party if it lands.
Debuff Ixion as much as possible with Expose and hit him with a full assault from your party.
Wear Ixion down with your most powerful attacks and use X-Potions to keep up with his debuffs. Once he hits critical, his damage will increase -- you can heal through it or hit him with a gil toss to finish the battle.