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.