On the mission Vendetta (sniper mission) after you go through the burning building (where the Germans are shooting flame-throwers) you're told to follow the Sergeant up the ladder to snipe the Germans below.

Once up there shoot the guy with the flame-thrower to start the fighting and after a short while 4 Germans will appear on a balcony bellow (left side of area).

Go back out onto the walkway (area after climbing the ladder) and aim down at them.

There should be 3 soldiers bunched really closely together. If you aim correctly you should be able to shoot through all 3 of them with one bullet.

And ta da achievement unlocked!