FIRST make shure every body also aeons have overdrive ready to go. go up to the boss and start the battle by makeing tidus use hastga. make sure yuna is on the feld summon aeons and make them use there overdrives if that Doseint kill the boss right away use the party`s overdrives. if that still doseint kill him if the boss isint immune to zombie make him a zombie and get a potion or cast a spell that heals infinity helth givecast it toat the boss and it should kill him.