Really?
Eh, volume two's final boss beat me once, but I just leveled up a bit and killed him easily the second time.
Sadly, G.U. has very little difficult in that way. Even One Sin and God Eater are pretty easy if you're the right level. Not even the ridiculous over-leveling, even just 3-4 levels often do the trick...
Good thing I don't really play it for the gameplay... ^^;;;