This farming method requires for you to be in Pure Black World Tendency in world 4, be in Soul Form, have at least 30 intelligence and have access to Firestorm, Soulsucker and Evacuate, Talisman of Beasts on the Right Hand, and Silver Bracelet + Ring of Avarice (female character required for Silver Bracelet)


Starting from the Old Hero archstone, run all the way back to the last Reaper of the 4-2 stage. Kill him with fast melee attacks and switch to the Talisman of Beasts. Once you're past the first doorway that you see, start walking slowly and auto-aim the Black Phantom Black Skeleton.

Once you're past the doorway that's guarded by the Black Skeleton, use soulsucker on him.
After defeating the Black Skeleton, switch to Firestorm and walk slowly down that ledge until you meet two Black Phantom Gold Skeletons. Approach the first skeleton to his side, and use Firestorm (Having a Kris Blade +5, Ring of Magical Sharpness and Old Monk Wraps helps a lot), it should kill both Gold Skeletons in one cast. 

Afterwards, drink a Fresh Spice (if you're only at 30 intelligence, you'll be drained out of mana by this time) and cast Evacuate. Repeat all over again.

This process can net you a whole total of 111799 Souls per 100 seconds that it takes you to do the whole process, once you streamline it, that's 3.8 Million souls per hour.