If you are like me and just want a real lazy way to clean up this achievement post campaign -
I fast traveled on the map (
) to Koani Charlie in the top left (Koani section). There is a Unidad outpost directly South West down the mountain. Stay high on the rocks and you can just pull out you binoculars (
). There are a good 10 or so walking around and in the sniper towers. After you can’t see anymore to mark, pull out the map and fast travel back to Koani Charlie. Rinse and repeat til it pops.