*Update: Credit to Catatonic Nali for posting a video of the solution, written out below:
I got this easily during the Soviet Installation section, Alone Again mission, when the objective is, "Climb the copper mill to reach the mine entrance".
As you climb the mill there will come a point where you take a ladder to the top of a water tower and have to jump off toward a red building and grab the ledge. Listen for the dialogue: "(via radio) Yeah, looks like we found a jackpot up here. All sorts of artifacts. It matches the description of the Soviet intel".
Jump to the ledge and watch the guy with the walkie talkie. After just a few seconds you'll see he stops at the edge of the platform, facing the other direction. Climb up, approach, and fire your gun next to him to alert him (don't hit Y immediately or you'll get a stealth kill and it won't count). Once he turns around and the insta-kill Y prompt is gone, just tap Y once to melee him off the ledge.
As soon as you get the exp for the kill just reload the checkpoint - you'll be right back on the water tower. Takes only 30 seconds for each one, so just a couple minutes in total.