This achievement reminds us of why AI partners, while okay, are no where close to the real thing. This is the only achievement in the game, that I can remember, that you need a second player (or one person with two controllers). The AI partner will NEVER pick the shield.
You will need to be behind your co-op friend when he has the shield and you will need to kill a total of 50 enemies (as the achievement said).
This achievement also is the one you most likely will not get during the course of the game, unless you put a major focus on it.
My favorite spot for both of the shield achievements is on the Aircraft Carrier. As you progress through the level you will reach a check point and will see a riot shield in the corner of the same hallway before going outside on a narrow walkway. There will be several enemies come out at you, once you kill them all reload the check point (DON'T GO INTO THE DOOR AT THE OTHER END OF THE WALKWAY). It will only take about 4-8 reloads depending how many you have killed in the duration of the game.