Requirements for the build: 1. As close to 59 mana per attack as you can get. Higher is even better since then Spirit Barrage is free to cast. I have 82 mana regen at 1.4 attacks per second, which equals 58.57 mana per attack. 2. 20 pick up radius (Yay Thing of the Deep).
How the build works: Rush of Essence gives 49 mana over 10 seconds for any use of a spirit skill. The spirit skills I'm using are Spirit Barrage, Spirit Walk, Mass Confusion, and Soul Harvest. Spirit Barrage costs 108 mana, but with Rush of Essence, it essentially costs 59 mana. With high mana regen, it's either almost free to cast, or actually gives you more mana over time as you cast.
Why to use this build: You can farm incredibly fast by using Grave Injustice to lower cool downs and Spirit Walk - Jaunt to sprint across the map. Zombie Bears and Spirit Barrage - Well of Souls are both very high damage spells. You can actually hit one target with all souls for 425% weapon damage using Spirit Barrage - Well of Souls. See the link below for info. This is great for killing Key Wardens, Bosses, Ubers, and elites.
For those with lower mana regen: If you have no trouble staying alive, you can use Blood Ritual instead of Spirit Vessel to lower the cost of all spells by 15%.
Idea for the build. This also shows how to hit a single target with all souls.
Profile: