I discovered (accidentally, when I started the game on Insane) that you can do this on the tutorial mission when you start a new game. In the very last checkpoint of the tutorial mission (when the E-hole is introduced), have both of your characters run as far head as possible. When the grubs emerge from the E-hole, have the character that's slightly behind shoot one grub with the pistol (and disable Overwatch for the character that's ahead if needed). Simply have the character that's ahead move toward the bottom of the ramp and the mission will complete and the Achievement will pop. No need to kill anyone.