After farming act,1 on Inferno, and finally beeing able to solo “Zoltun Kulle” and “Belial” with 5 stacks of nephalem valor, I can proudly present to you the fastest way to get it done. At the moment it can take me from 35-45min all depending on what elites I find..
The drops from Belial is a lot better than then ones from the Butcher (surprise surprise..)
So just choose the quest: “8.The black soulstone, realm of shadow.” And take the waypoint to Zoltan Kulle’s archives. Start grinding for the nephalem valor, keep destroying objects, kill the two guardians to get his body, kill zoltan kulle, kill the sorcerer on your way to belial, kill the elite pack in front of belial and kill belial. If all goes well, you might get some nice upgrades for your gear.