Some are probably wondering how to powerlevel and boost characters after Azmodan has been hotfixed. Well, this seems to be the fastest method with the lowest effort and risk involved. Just kill Zoltun Kulle in a single player game, then bring the other players before approaching the black soulstone.
Quest experience is ~35k per run on nightmare and ~95k per run on hell. The character that does the boosting gets an additional ~16k kill xp per run in hell. Easily soloable with a Demon Hunter that is still leveling. The one that does the killing in this video hit 60 just a few runs before. On nightmare, you just namelock on the Boss and hit Smoke Screen once you lose life = done in 5 seconds. On hell, see the video. This was done on hardcore mode, so as long as you somewhat dodge his spells (the debris and the tornade do crazy damage), you are fine. I still recommend to dodge the fireballs, but you can also eat them with some smokescreen+preparation+pot+globe rotation.
To increase the gain, do this with multiple characters. Since they can be in town all the way, you can easily boost your second account this way, like I did for my third Demon Hunter in the video.
If you are three people, one can get triple xp, one double and one normal. To do that, right after the kill two players leave game and click "resume" independently. The remaining third person can turn in the quest. Then he joins the game of one of the others, they turn in again and finally both join the third game for the last turn in.
With this method, it is easily possible to go from 50 to 60 in ~4-5 hours for three characters, if you rotate the triples fairly. If you boost one, it goes even faster. Since the runs are so quick, this is even better xp/hour than Azmodan in my opinion, but a lot more stress because of all that joining and leaving.
I publish this because I believe in full disclosure and I want it to be fixed. In hardcore mode leveling _does_ matter and if you are forced to play through hell, the risk is a lot higher than with such cheesy methods. Especially with inferno tuning and future hc pvp, leveling should be a little challenging and not a stupid, repetitive and extremely boring process like this. At least a regular playthrough should be the more efficient way to get max level.