This is the Zunimassa build that I use. It is for the 70-75 solo greater rift range. It has a high amount of survivability due to Zunimassa set (69%), Lakumba's ornament (60%) and Unity (50%). It allows for no worry about casting your Grasp of the Dead due to zero mana cost and zero cooldown. Just spam away! Easy! I have cleared GR79 with it at paragon 700 in 14:30 with a not-optimal rift.
The basic playstyle of the build is to allow your Gargantuans to do the majority of your damage. Keep track of them and cast Grasp of the Dead on whatever they are fighting. Try to group up enemies using Piranhado for efficiency. Don't be afraid to run ahead and pull your Gargantuans to you if enemies are spread out. I like to run ahead and wait for the Gargs to pop behind me, then cast Piranhado on top of them if there are enemies nearby. That clears stuff fast. Just try to stay close to the kill zone for the Grave Injustice proc to keep your cooldowns available.
Gear / skill changes:
You can change the rune in Soul Harvest to Soul to Waste (+5% movement per stack) once you get comfortable with the survivability of this build for added movement speed = faster clears.
You could likely switch Wilken's Reach in the cube out for furnace for pushing higher GRs. You would just need to be more disciplined with your GotD casts.
One could also switch out a well rolled pair of Compass Rose / Traveler's Pledge and replace the Unity / Hellfire combo listed above to push higher rifts.
To make this a group build, replace:
Unity with a Zuni ring
Convention of Elements with Ring of Royal Granduer
Shoulders with Helltooth shoulders
Any other 1 Zunimassa piece with Helltooth.
This gives you 4 piece Helltooth and 6 piece Zunimassa. Same damage mitigation (trading Unity for the 4 piece Helltooth bonus) but slightly less damage due to losing CoE ring. Still works great though!