Well, I can't help you from a nb's perspective.. But as a stam DK, the key to beating them is just wearing them out while keeping your distance. I will also say there are certain fights that really just aren't worth fighting. Players who stack nirn, have sharpened weapons and crazy mitigation really aren't worth my time. I focus the players who are effectively doing damage and then if the derp with his shield up is still standing I'll see if I can't take him out.

EDIT: This isn't saying I'm not capable of killing tanks in PvP, I'm just saying why focus the guy who's just frustrating you, when there are players killing other players.