o gawd. the underwater guy, right? jebus...
Indeed...and with Oniya being a completionist..she's going to be tearing her hair out in anger over it. ^_^
Let's just say this, Oniya..he's stronger than the final boss. A LOT stronger...and if you don't get a certain item beforehand, you'll also be on a timer during the fight...a timer which will definitely run out before you even make a scratch in his HP.
Of course, if you get said item, the easiest way to beat him would be to cast Knights of the Roundx4, Which would probably kill him out-right...but then again, that's the entire reason you're fighting him in the first place, to be able to get that Materia! HA!
...So yeah..me thinks Oniya's going to have some unhappy times ahead when she meets the weapons.