Brütal Legend

This is a pretty simple achievement. Go to Multiplayer and choose "Practice AI" and put the enemy on Most Gentle. This is pretty much a cake walk. I put the enemy on Tainted Coil and chose Ironheade (edit: using Tainted Coil makes this achievement almost impossible to get*) as my army because I'm more comfortable with them. I then chose the bladehenge level and proceeded to call a rallypoint, picked it up, moved to the first fan-geyser and spawned 2 sets of headbangers, 1 set of blade girls, and 1 bassist. I captured both geysers and upgraded my stage twice during the small fights. I then spawned two roadies and let them wail away on the enemy stage while the enemy was attacking my merchbooths. Be sure to use Battle Cry a lot to strengthen your units. Easy peasy lemon squeezy.

Things you are allowed to do:
-Solo for a rallypoint flag
-Solo for all units to gather around you
-Solo for buffs (battlecry and dawn)
-Solo for merch booth

Things you CANNOT do:
-Attack in any way including the solos for face melter, anvil, and zeppelin.
-I wouldn't even touch the attack buttons, even if you aren't near an enemy.
-Double-team even without attacking.*

*Edit: Ellusion no has pointed out that you cannot double-team with any units no matter what, making this achievement almost completely impossible with Tainted Coil.

Play as Ironheade and go against Drowning Doom on the Battered Bluff map. First thing, Upgrade your stage once. Now wait until you have enough funds to build some fire barons.

Make about 4 groups of fire barons and use LB to target the enemy stage and go to your troops and use -> on the D-pad to make your guys go to the stage and attack it.

While they attack, use the battle cry to make them stronger. Just wait at your stage while they work and it will be over soon.

The trick to this is that the other team is busy getting merch booths set up and it is a big map. So they will be busy doing that with low level troops and you are free to pwn that other stage!

This trick also works for the "Iron Fist" achievement for winning a Brutal AI battle. But be sure to attack the other team's leader to keep them off of your fire barons. Also, dropping a zeppelin on the stage helps a ton too.

Participate in a battle vs. an AI opponent. Do not engage in combat with enemy troops using your avatar; let your squads do the work. You can still play solos to create merch booths (or their equivalents). I did not try any buffs or damage-causing solos, so your mileage may vary with those.