Rogues are one of the hardest things you can duel. All melee besides rogues are the easiest, you can pretty much just kite them the entire duel and kill them with bleeds. Assassination rogues are just flat out broken and probably shouldn't be beatable until they get fixed. Outlaw relies heavily on Roll The Bones RNG and the duel outcome will probably be based on that.
The impossible matchup is Affli Lock. For pretty much any melee.
They basically stand still in their aura mastery circle, put dots on you and spam drain life. The drain life will outheal all your damage, you can't kick it because they're standing in a circle that gives them aura mastery and you just /forfeit. There's nothing you can do. Only thing I've seen beat them is sub/outlaw.
Other than that, most other casters aren't much of a problem. Fire mages are probably easier to beat than they were in WoD, which is really the only caster that could beat it that expac.
Oh, and Demon Hunters are pretty annoying for obvious reasons with their stealth vision. If you're a nelf just meld and reopen or even pop incarn for restealth if you really wanted to. They're not that bad of a matchup, I think feral should probably win but not by a big margin.