Basically, you have to kill 4 enemies with 4 shots during a slow-mo breach in Single Player or Spec Ops. In the video below this is done in Single Player on the mission "The only easy day...was yesterday", on Veteran, although difficulty does not affect it. You can kill them in UNDER 4 shots, but not exceed the four shots. I prefer not to use the M203 Grenade Launcher because this is the first chance you have to get this achievement and the presence of hostages makes this difficult to aim.