The REAL easy way to get this achievement (if you are connected to Xbox Live, thanks ego orb) is by going to Multiplayer >> Custom Match, then selecting Tainted Coil and starting a match without any other players or AI. Capture the fan-geysers and do not attack the empty enemy stage. Upgrade your stage. The Tainted Coil spawns units through the stage, battle nuns, and war fathers.
Shouldn't take too long and it's easier than the other solution posted here because you don't need to have a friend with you.