This is the only single player campaign achievement that can not be gotten without a second controller/human partner. You have to be the one behind doing the shooting while your partner holds the Riot Shield to get this. I recommend doing it on Mission 2, Afghanistan on Casual difficulty. There is a part in the level where you are in some corridors filled with gas. There's a riot shield right outside the lift you come out from. Have one player pick up the riot shield and one behind doing all the shooting. Take the left path and circle clockwise. You can easily get at least 12 kills before the shield breaks. Once it breaks, restart from checkpoint and you will be outside the lift again. Do this 4-5 times to get this achievement.