Trying to get Donnell up to speed makes even normal mode more of a challenge. I've got to whittle down enemies to just enough HP that he can still finish them off, while ensuring there's no other enemies in range to take a shot at him. And nobody dies. And everybody sticks close to whoever I want them paired up with for maximum hearts.
Everything's gotta be perfect, all of the time.
I just paired Donnel with Kellam for the defense boost and had no problems leveling him up.