This achievement requires the "Insurgency" DLC pack.
You can get this achievement either by yourself or in co-op with a buddy. Difficulty does not matter, so might as well make it easy and go on rookie.
If you're doing it by yourself, go into Deniable Ops and pick the "San Francisco, CA" map on Hunter.
The idea is that there are shipping containers around the map, you have to kill 12 enemies while both you and they are in the container. I found the easiest way was to hide in a container and when a guy wandered near shoot at the wall inside the container. He'll come charging in, then just melee him dead. Repeat 11 more times. Also, it does not matter if you are detected (in fact, if it did this achievement would be near impossible). On my successful run, I was detected after killing the first guy.
If doing it in co-op, follow the Achievement Hunter video below.