You can do this in the first battle. it's almost impossible to miss it, unless you either die or force the enemies to split..
-So what you want to do is wait for three turns.
-After the THIRD TURN, the cannon will be online.
-Normally on the left side you should have 6 enemies grouped up and attack with the cannon to get the achievement (including the robot, i've done this twice now on the hardest difficulty and i got it)
you can do this in about 5 minutes, considering it's literally the first battle in the game.