Props for fighting 90's, but this is what we used to do on our 80's in Cata Vs 85's at the start, now we used to win randomly here and there and most of the time it was because the 85 didn't take our twinks seriously. We had a Disc Priest that done nothing but duel 85's. He won pretty much all the time.
As soon as someone took him seriously though, he was screwed, Ie. Me on my 85 DK. I knew how potent his healing and bubbles were, so I just nuked him in a silence. I imagine if the 90's you're duelling were remotely good geared or even taking you seriously you wouldn't stand a chance. They just have WAY too much base damage nowadays.
Still, I don't mean to diminish your achievement by any means, I just believe it's not viable to duel 90's on an 85.