Farming Zoltun Kulle is going to be some of the best XP you can get on Act 2. With decent gear it takes about a minute and a half and rewards you with 10150 XP, amounting to over 400k XP in an hour. Any class can go this, but if you want to do it really fast, get some upgrades from the AH first.
1: Choose change queste in the character select green and choose Part 3 ” Soulstone Chamber” of Quest 8 “The Black Soulstone” on act 2.
2: Go to the Archives of Zoltun Kulle waypoint and run straight over to the Soulstone Chamber portal.
3: Skip or Watch the conversation with Kulle.
4 : Kill Kulle and walk over to the Black Soulstone floating in the middle of the room. Skip the cutscene and go out of the Chamber the same way you entered.
5: Run back down and take the waypoint to the Hidden Camp.
6: Talk to Adria and you’ll be awarded with 10150 XP and 1120 Gold