Draco and Ernie
Draco was pretty nasty against Ernie, and he's well-versed in Dark Arts hexes. Ronnie, depending on what you want to happen, Draco can injure him in some way shape or form.
He's not particularly brilliant at blocking, but he is top notch when it comes to offensive hexes and thinking of them on the spot...as well as sensing weakness.
He wouldn't go as horrible on Ernie as he would on say, someone like Neville, but it would be evident that he's still not messing around or offering any freebies, especially being watched so carefully by the Carrows.