I'd have to say Sion, because I had to repeatedly use Battle Meditation and Force Heal to keep up with his pace.
It was extremely tedious.
Though Nihilus was too easy as well. That got me annoyed, too.
Let there be rock!
"I hope they realize that music is music, and that music is not a scene, not a style. Music is a beautiful ****ing thing to listen to. It is not a thing to ****ing preach to others about, it's not a ****ing cause. It is what it is—and that's a beautiful artform." - Synyster Gates
Check out my Fanfic:
Star Wars: Knights of the Old Republic II: Embers of Destruction