Tilted!

Winning a mounted duel ALSO counts as winning a normal duel as well, earning the "Duel-licious" achievement as well if you do not have it yet.

This achievement appears to still be bugged years later. The below is copied from 2014 and worked today 20/07/18
SorynAlleria on 2014/12/09 (Patch 6.0.3) To get it while it is bugged: Challenge someone to a duel near the horses. Have neither person get on the horse until the duel has officially started and the other player is red. Once the duel has started, the person who needs the achievement gets on the horse. The other person does not get on the horse. The other person types /forfeit and it grants the achievement.

this achivement is very bugged, when you challenge another player to a duel using the 6th ability on the vehicle bar when mounted dismount you and the other player as soon as the duel countdown reaches 3. Have submitted a bug report to blizzard.