Firstly, combat swords is hardly the best build. Essentially, it's a question of style of play, and even then, combat specs have serious limitations during PvP.
To the OP, mages are tough nuts to crack. I switch between stunlock and mutilate builds, which are usually effective anti-caster weapons. Except mages. I just can't sit them down, it gets frustrating. If after the 11-second burst (CS-muti-KS-muti-3/4 point finisher) they're not down, they either blink to the other side of the room, or nova me in place and wait for back up. True, I'll have them below 10% health, with possibly a few doses of deadly poison or a rupture, so they're going down sometime soon. But that's no good to me if I'm running back to my corpse myself. You don't get no HKs when you're dead.
Though I have had a few success stories. As always, get him on your own terms, do not let him know you're there with a distract. Don't know what your build is, but if you're a fan of the stunlock, put him in one and keep him there. So CS-backstab/mutilate-KS-BS/M-blind, give yourself a few seconds, maybe restealth and repeat. He's probably spamming nova or blink, so timing is the thing. That really oughta do it. Haven't yet figured out my sub build, so have no insight there.