Easily done in level 1-1 on Hurt Me Plenty. Don’t bother going in the room on the left unless you want the armor. Continue through the level to the first door. Kill the 2 enemies in the first room then go to the next. Kill the 1 enemy in the hall before the zig zag walkway room and the guy at the end of the walkway. There’s a monster up on the wall in the corner that you need to take care of as well. Continue through the next door. Kill the monster in there and finish the level for the achievement.