I don't know how far you are into that game, or have you completed it before, but I died dozens of times in FF X-2 against the Magus sisters, I have heard that it's the last major battle, I don't know this, because I never beat them... If I remember correctly the place where they are fought, doesn't allow leveling anymore, and If you enter that place with maybe few levels lower than is the optimal, you're in for a big trouble. I think in most Final Fantasies there has been opponent that is very hard unless the player has done some extra work for leveling up( other than just the battles during normal travel) I can't think of any other hard battles in FF X-2 though.