1 very good mage who can spell steal tank on HK, a few hunters for misdirects, couple warlocks to enslave pets of HK's warlock and tank him with them.  rest doesn't matter

Ranged is best here because HK whirls, gruul has cave ins that deal a lot of damage and are normally in melee range. But he 1 1 mob and doesn't leave the tank unless it dies