Brütal Legend

I did this with a friend online. It doesn't matter what stage you play on, whichever one you want will work fine. Have your friend play as Ironheade while you play as Tainted Coil. Capture the fan geysers to get resources and upgrade your stage until you are able to select the Pain Lifter unit (I think the War Father spawns him). Once the Pain Lifter is spawned, have your friend spawn 4 squads of headbangers at his stage. Instruct your friend to make a mosh pit with them all and stand still in a circle. Double team with the Pain Lifter and drive over to your friend's stage. Use the Pain Lifter's special double team catapult attack and put the targeting reticle right over the mosh pit. Then, fire. If you did it right, you should unlock the achievement.

Note: If you fired and the achievement didn't unlock, try firing the catapult and then pressing the fire button again right when the agony ball hits the mosh pit so that it explodes.

You can do this on your own.

Make AI Practice game, play as Tainted Coil against Most Gentle Drowning Doom on Feeding Ground.

Capture the geysir on the right and keep defending it until you can make a Pain Lifter (or an upgraded Pain Lifter).

Keep killing Ophelia as much as possible, so she can't upgrade. That way she will only spawn basic units. At some point she will spawn 3+ units (18 single units) of Gravediggers and keep them at the stage.

Place your Pain Lifter on the path between her stage and your geysir. Ophelia will eventually advance with her 3+ units of Gravediggers against your geysir. Double team (by pressing Y) and fire the Pain Lifter and make it explode a little above ground by pressing A.

Hopefully you will be able to take out all the Gravediggers in one blast. If not, just keep trying. The clue is to keep Ophelia from upgrading to more powerful units or veichles.

This worked for me at least.

(Same tactic can be used for Ringleader and Painkiller, using Ironheade against Drowning Doom, or Dollpocalypse using Drowning Doom against Drowning Doom)

You can do this by yourself easily enough.

First, make sure you are playing on the Dry Ice Quarry and then set the AI to Drowning Doom on Brutal difficulty. Don't worry about creating any units or capturing geysers, just keep upgrade your stage and create a Warfather. Use the Warfather to summon a Pain Lifter and then wait. Keep an eye on the enemy and destroy any unit that aren't Grave Diggers. Once the enemy has captured all the geysers they should start heading towards your stage.

Since you focused on destroying all the single units, they should have at least 3-4 groups of Grave Diggers. Double team with your Pain Lifer and focus on the largest group. If there aren't enough together then wait until they reach your stage and group up. Launch the Agony Ball, roll it into the middle of the group, and boom! Achievement unlocked.

This method is also effective for the 'Dollpocalypse' achievement.

If you can't get this done with a friend on Live for any reason this can also be done against the AI multiplayer. Set AI to Brutal difficulty as Ironheade on Bladewoode. Upgrade stage ASAP then recruit Warfather then the Painlifter, simply wait it out till you see a horde of metal heads coming your way. Double team with Painlifter & direct the Agony Ball into the middle if the group, detonate and presto! Bits of headbanger and hot chicks everywhere, oh and an easy 15G ;)
Credit to HarvestNine from Achievement Hunter. Great solution, worked first time.

If you are doing with a friend and both need the achievement, you can play Tainted Coil vs Tainted Coil in any map and unlock for both in the same match.

Just build a lot of Soul Kissers in front of the stage for the other to hit with the Agony Ball.

Very easy. Did like this on the first try. Hope it helps.