I actually used the two earlier solutions (from Mojo I Uk and Br1Zzo) to help get mine, but I believe that they are each missing a body part . . . the stomach (hence the comment in the video that "apparently, that first torso shot didn't count").
Like they both mention, I got it on the level Friends From Afar and here's the strategy that I took to get it:
1) Do NOT use your thermals while you are firing. Particularly when you are trying for the body shots, it is often much harder to figure out what part of the body you are aiming at with thermals on.
2) I believe you need to hit 5 different areas:
- head (you will see the headshot symbol on your screen)
- chest (spotter generally calls out "center mass")
- stomach (spotter generally calls out "gut shot")
- arm (spotter generally says that you hit his arm or says "winged him")
- leg (spotter will sometimes say that you hit his leg, though I've heard various other callouts)
I don't think you need to hit the left and right side for the arms and legs, but if you have trouble getting it to pop, it certainly couldn't hurt to try. :)
3) Go for the obvious body parts when looking at your targets (i.e., don't "force" a particular shot, just because you need it). There are plenty of opportunities for each body part:
- if the guy is standing with his legs apart, go for a leg shot
- if he's crouched, holding his gun out, go for the arm shot
- save your head shots for the snipers
- chest and gut shots are often best for the guys who are popping in and out of cover, since those are the two largest body targets
Those are just examples though . . . you may find better opportunities for each part, depending on your shooting style.
For the record, I tried 2 or 3 times with my thermals on, hitting, what I thought were all of the spots that I needed to, but it was was hard to tell visually. After watching the video that Br1Zzo posted, I tried it without any optics on and it popped during the second group of guys. I don't think using thermals keeps you from getting the achievement (though, I suppose it's worth exploring), but it was easier for me when I didn't.