Materia Keeper had me when I was a kid, but I soon got past him. That Seymour battle on Gagazet was tough, but I was expecting it (yay for spoilers). I've never really tried to defeat optional bosses, so I can't really comment.

But the two bosses that still have me beat: Adel and Seymour (inside Sin)

I'd be able to beat Adel now if my disc worked, and I'm too lazy to level for Seymour. So they'll be on hold for a while.