Never change to other parties. That way you won't get stuck in the situation that the second (and maybe third) party will just keep killing the head and the arm and you never get to attack the main body or the core. From what I've noticed, your first party can always attack those parts that you need to kill to defeat the boss. If you have KotR, cast it once or twice to win the battle. Omnislash or some other limit break that hits multiple times may also do. If you don't have them, keep hammering the body and other body parts until the core starts to take damage, then attack the core.