I found this to be fairly easy on Terrorist Hunt with the Spas on Villa.
Set the difficulty to Casualty, with high enemy density and lone wolf turned on (You don't want your team stealing kills). When the level starts, move over to the wall on your left and look out into the opposite garage. There is a terrorist behind the car in that garage, kill him and remain behind the wall. Enemies will now rush down the stairs on the left and the ones in the opposite garage and you should easily be able to take out 7-8 guys without moving from your spot. Once the enemies have stopped rushing you and you're sure that the area is clear, walk up the stairs inside the other garage, fire a shot at the metal staircase inside and you should hear the enemies rushing again. Straight after you fire that single shot sprint back to your spot in the garage and pick them off as before.
Rinse & repeat after you die.
I got the achievement in less than an hour using this method, having not used shotguns much in Single Player.
Caution - Occasionally, enemies will flank from the right inside the garage and catch you off guard, usually there is only 1-2 so it's easy to turn around and kill them before they kill you.