Och it's simple. (Well i thought so) Basicly do NOT! Under any circumstances use Aeons unless they have an over drive or Yuna has hers so you can use it as your first move. Secondly as soon as it uses beserk tail use a remedy or Esuna on the afflicted because if you attack it it'll counter hitting whatever 3 circles are infront of it (Usually the ones your 3 characters are standing on) Thirdly. Make good use of the "move" command. You need to get all 3 characters behind the thing before you attack it either physically or magically or it will hit your characters (You don't have to get all of your characters behind but it's better so that you can keep hp high) Make good use of your Aeons overdrives and your characters as i don't think the boss counters them and it should die pretty quickly depending on how well trained your characters are.