Easy Gamon Will Save Us! Achievement and/or easier Nazgrim kill.

Nazgrims resources are classified as mana(not as rage for whatever reason), and it only goes to 100.Black Grasp of the Destroyer and Soul Breaker both still have mana drains on them, and is capable of keeping his rage drained and prevents his warsong ability.

I have tested it and it works.
Does not work on Immesues and Sha of Pride, as their resource bars are not classified as mana, and I have not tested it on anything in the second half of SoO.