There are loads of heartless bosses throughout the Kingdom Hearts Series that are heartless, and you have no idea about most of their nobodies. Clayton, for example.
I'd save Demyx. He never was much of a fighter by nature, you could tell by the way he complained everytime he had to fight, but poor Demyx met a fighters end. The poor guy never stood a chance.
I found the Hades Paradox Cup, round 49 WAY harder than him. Even at lvl 99 it was a struggle, cos you have to watch out for 4 people, rather than just Sephiroth.