Summon the Battle Cry to free the Headbangers.
Once you are done learning how to command your Headbangers, have them destroy the blockade that leads deeper into the Bowels of Hell.
As you get towards the last section of this level there will be more Hair Metalheads for you and your troops to deal with. Kill them off, including any nearby Bouncers to have the chance to go face-to-face with the Uber Bouncer.
For starters, keep your distance from this boss because he doesn't have a range attack.
...and then use Earthshaker to drop the hot coals on top of the Bouncer.
Use the Battle Cry near the headset to summon everyone Headbanger to your side. After this, the mission will be complete.