Pretty simple, Wyvern Sting, has a delayed DoT effect, if you drop someone to 5-6k hp range in a duel and wyvern sting them to sleep, then break the sleep with killshot (to guarntee the 1hp end of duel) the dot component still triggers and kills them =) you just have to make sure you finish them when you break the sleep and dot doesnt trigger before duel ends.
Enjoy
edit: The trick itself is the delayed DoT portion still triggering, perhaps its doable for other classes if you have a delayed DD/Dot component spell, can't think of any off top of my head, but I'm sure someone will