Like mentioned in topic, Old Kulle method is working again! At least on Normal difficulty (NM got nerfed XP, don't have friends online to check Hell, and my alt account don't have access to Hell yet).
So what you have to do:
A - booster
B - Leecher
1. A creates a game on Act II Quest 8, part 3 (enter Zoltan Kulle Chamber)
2. B joins.
3. A kills ZK. B stays in the city.
4. After ZK is dead B leaves game clicks "Resume Game", run for Black Soulstone, TP back to city, Turn quest in, Join A's game.
5. A take Soulstone, B leaves game.
6. B resumes his own game, turns quest in, joins A's game.
7. A leaves the game, B turns quest in, A clicks "Resume Game"
8. B Joins A's game, they both turn quest in.
9 Repeat from point 1.
1 Kill of ZK = 4 turn ins, it can be of course done with more ppl, so it's possible to boost max of 3 ppl at once.