At a point in the game you get Combat Time Pause Farts, kill all the enemies but one and empty his health as much as you can without killing him when it's your turn do the Time Pause Fart go in front of him and kill him. The achievement should unlock. Dosen't matter if it's one or many enemies but it's easier to do when there's one enemy left with low health punch it once or twice and be sure he dies during the pause.