Furthest I've got doing that was BGH251F2 (Iron Clad), and that thing thrashed me with Beam Cannon!

I also tried going through Ultimecia Castle, without Junction, didn't work. Catoblepas and Red Giant beat me left, right and centre.

Ultimately however, it should be possible, with the right amount of training. Just try and get your self to level 100 on the first disc then try. Anything IS possible, after all.