Thread:Everyl/@comment-71.74.255.0-20131022044658/@comment-4632427-20131025185040

You should be able to get Army of 12 during a Boss Strike. Just start a battle with exactly 12 Troopers and nothing else.

For killing an army of 12, the hardest part is finding an army of 12. I missed my chances in easy mission battles, so I had to do it by fighting Silver Wolves in Recoil Ridge. One of the formations they sometimes use has a total of 12 units. Rocks count toward the 12 units, but don't have to be destroyed to get the achievement. The formation can be cleared quickly with well-leveled checkerboard-strike units, like Rocket Trucks and Dragons.