Whilst going through the Soviet Installation, after climbing a red silo, you will find a lone guard talking on the radio. He walks over to edge and you need to melee him. Don't stealth kill him
. Walk into him and wait for the red triangle to appear above his head. Press
to knock him over. Pause the game and reload the checkpoint.
For those that missed it in the main story, this can be reached again by playing the "Copper Mill" level within Expedition mode. (Thanks to Rathelon
You can also use this for the five leg kills.
Also, just after here is a couple of rooms with several guys which you can kill with the revolver for another achievement. Don't reload until you get 6 kills. 6 headshots should not cause you too much difficulty here.
Here is a video if needed;