Gamon will save us!
Okay guys our raid got this tonight completely by accident!
The trick to do this is to release gamon and do trash as normal. Once you get into the tunnel and upto the warlock trash with the giant demon simply jump off to the side and skip it. Gamon will charge in by himself and start fighting the trash while you continue on. Simply clear the rest of the trash and then kill the boss.
Gamon will not be in range of any of Nazgrims abilities and will not die to the trash as they take FOREVER to wittle his health down.
We one shot the boss and were all shocked to see the achieve pop up. Gamon meanwhile continued to battle the warlock trash as we trotted off to the next boss