Super easy gameplay : you don't have to cast your Locust swarm thanks to Wormwood in cube. Just spam haunt and soul harvest big packs
Gargantuan is totally optional and up to you. Humongoid benefits from cold damage bonus. A few other possibilities: Mass confusion or Hex for more CC, Big Bad Voodoo for even more damage.
For speed T13 rifts / gem ups / bounties :
- change soul harvest rune to "Soul to waste" and horrify rune to "Stalker"
- change passive "spirit vessel" to "Fierce loyalty" for more speed
- change Unity to Avarice Band
- change Esoteric alteration to Boon of the Hoarder