I got it today on T6 after about umpteen times with GAS 501st. Took me a long time to figure out the right pattern of attacks to get them all low enough to have three knocked out with one attack.
When you used MM did you kill 4 enemies with one attack or 4 within MM? If you didn't kill them at one time I would assume that the game is reading each attack as one turn, but I could be wrong.