The challenge in this fight is avoiding the Spectral Keeper's counterattack after your innitial attack. It's important to position most of your party out of harm's way before attacking the boss. Start with a party of Tidus, Yuna, and Auron. Equip everyone with armor bearing Berserk Ward or Beserkproof if available.

Cast Hastega immediately so the characters can move out of the way before the boss finds them agian. Move two characters to circles behind the Spectral Keeper, and leave the charcter with the highest HP out in froont to keep the monster distracted. Otherwise It will turn to attack the entire party. have Yuna cast protect on the frontline person, then let that person attack. The boss will then counter attack striking the three cicles in front of it. It mmight also use its Berserk Tail attack, berserking the target.

Glyph mines will form on the surface of the circles, and thier elxplosions mean certain death. Check the CTB window to determine when each mine will explode. use the trigger command move to escape the explosion.

Since aeon's cant escape glyph mines, they die quickly. IF they are summoned, they'll only have time for one overdrive. So, summon at your own risk

omfg i most be overlevvled cuz i beet yuna;eska in one try aswel. i thought dont worry about healing your characters just keep one alive i men come on she only take like 200 hp off at the most. shes nothing REALLY EASY
the only time you should heal some one is if there dead i mean if there all at 200 hp then use a mega potion but seriously dont worry at all about being at lets say 600 -max hp. JUST MAKE SURE ONE PERSON IS ALWAYS ALIVE. then you will beat her for sure. dont waste all your potions and hi potions or all of yuna MP.
ITS SO EASY OMG I CANT BELIEVE YOU HAVNT BEATEN HER

Please don't double post. ~m4tt