I got this at the SpecOps mission Gulag. I believe it's Charlie ops. If possible, do Co-op. Makes life easier.
Breach the wall, make sure you survive to the second room. Don't forget about the bastards on the sides above you.
If you run a little into the next room, you spawn the enemies with Riotshields. Walk back into the second room. If you have a buddy with you, let him draw attention, and you smash his back. Pick up the riotshield, draw attention. Beat a normal enemy once, and a riotshield twice (first smash frontal, walk behind him while he tries to get up on his feet, second smash behind).
You're welcome ^^