First go into battle with FULL overdrives (including the aeons). I don't remember what level i was on but it did take a lot of training.(CAn you have a level???) Anyway, make Wakka defeat the negator and then make Yuna grand summon an Aeon (preferably Ixion, because lightning does 1.5 times more damage) the crawler will do a mana beam attack, which MAY kill your aeon, but it didnt kill mine so i got to do a second overdrive. if you manage to do everyones overdrive it should die.