Easy, when it protects itself with the tentacles, just use magic. And attack when it's unprotected. If you still have trouble you should train your characters a bit more on the fields. :)

hey well the best way to beat that thing is to focas on the head and tha middle of the creature. use lulu's magic, and una's summons, and wakka to do the damage to the head and the magic for the middle as well. auron and kimari have peircing weapons so thier best for the arms. if youve run from alot of battles, i recomend starting over and never running from one. it makes the boss battles easier and makes your level high compaired to others. granted you take time to level up from time to time