I came across this today while doing random duels with my hunter friend.
First, you must be specced for Wyvern Sting.
Here is the tooltip for Wyvern Sting (taken from wowhead.com):
Wyvern Sting Rank 4 255 Mana 35 yd range Instant cast 2 min cooldown Requires Ranged Weapon
A stinging shot that puts the target to sleep for 12 sec. Any damage will cancel the effect. When the target wakes up, the Sting causes 942 Nature damage over 12 sec. Only one Sting per Hunter can be active on the target at a time.
1. Start a duel with the person you want to kill.
2. Get him down to very low hp.
3. Shoot him with Wyvern Sting.
4. While he's still asleep, finish him off with one hit (preferably aimed shot, just to be sure it brings him down to 1 hp).
5. You will win the duel, but then the DoT from Wyvern Sting starts after the duel.
6. The DoT ticks once and kills him.
Have fun.