A lot of people still don't know this so I'm just spreading the message.
1) Click on Public Games
2) Make sure your Monster Power is set to "No Monster Power"
3) Select Act 1, Quest 9 "The Imprisoned Angel" in Inferno
Join that, and you'll find people constantly dueling.
(Occasionally you'll find people actually farming or trying to complete the quest but 90-95% of the time it works.)