Another simply way to get this is in the Manhattan Nuclear District, through the kill board/death card mission. As Wolfie McG stated, if you want to speed this up you can use throwing hatchets but make sure you hit the guy and not his equipment.
1) There are two enemies in the room right in front when you start, stealth kill them
2) When you crouch out of this room WAIT until you see the guy on your left walk past
-Sneak kill this guy (guy that walked from left side)
- Then sneak kill guy in control room, should be ahead of you
3) After killing the guy in the control room, turn around to the hallway where you came from and follow the right wall for a constrictor tunnel. Once in the tunnel you should see a guy and a armored guy. Wait for the yellow guy to come close then turn back around, this way the armor guy doesn't see you. Stealth kill yellow guy
4) Now immediately after the kill go down into the half circle with the computers (armor guy won't see you down here as long as your crouched). Once you reach the end STOP and wait for another guy to walk in and go against the pillar in front of you. Once he's against the pillar sneak kill him. Then continue up elevator
5) Make sure you save at the top of the elevator. I wasn't sure if you had to do this without being seen but this is how i got past this part. Wait by the shifting doors and when they open crouch and walk to the left. Stay hugging the left corner of the rail cart and only move out when the armor guy is walking to the center. Now go under the rail cart in front of you and wait for the flying robot to pass. Once it passes crouch and go through the rotating doors in front of you.
6)Now when you get to the ramp leading to the next open area with the last signal do not run into this room. Instead, crouch and walk to the right side of the entrance and shoot your silenced pistol at the other side of the room (make sure its at an angle so the guy walks in facing away). Once the guy walks in stealth kill him.
7) Same area! Now move to the left side of the door and shoot your silenced pistol towards the center floor of the big area. IF the other guy on the left side is on your level he will walk to the center of the room and you can sneak out and kill him.
8) 2 MORE!! Crouch walk up the ramp on the left side. Follow it until you reach a ramp leading to the next level. Once on the ramp turn around and, if you have the double jump ability, climb up the two ledges to the next level. Once on top stay crouched and head to the constrictor tunnel on the right wall and do not exit it. Wait to see where the ubercommander is (he should be right outside the tunnel). If he is not standing to your left then head out and he will be to your right. Kill him......like a slippery snake
9) LAST ONE!! Now there will be a guy walking to the railing to your right. So after killing the commander go straight ahead to the wall and wait for a bit (you can corner lead on the right of this wall to see where he is). Once he passes you and goes behind the wall use the constrictor tunnel and stealth kill him once he's on the railing.
If for some reason you don't get it, then roam around all sneaky beaky like and chuck a hatchet as someone (there should be a guy on the railing across the room if you look over the railing where the 10th kill was)