I'd say Xaldin, but he was not that hard for me, as it only took two tries for me crack down on him. (Yes, you can run out of Mickey's saves heh) He wasn't that tough, you just needed to be fast and good at dodging crap. I dd very well when I beat him, as I kept my distance and made sure I wasn't in a bad position, as his combos are the worst. Healed when I needed to, at a distance usually, and hit him with Master with two bars left. Only needed Mickey once too.
Second up, which at the moment was the hardest....would be Xigbar. Man, he took some time to take down, as I was dong fine until I got down to his last two bars. There he went crazy with his blasts, standing shooting them in a mad frenzy. That attack would ALWAYS nail me in the air and I'd be helpless. No one would heal me either, so I always lost it there. Kept trying and the same thing happened. Eventually, I got annoyed and went from level 40 to 48 and came back, got to the last two bars, used Final, and owned that eyepatch's behind when he went berserk.