This achievement requires you to kill an enemy from over 70m with a bow.
The way I did this was waited until I got the ability to craft explosive arrows (crafted with 1 grenade). Once I had that, I went to go look for an outpost where I could get a decent shot on someone.
I'm sure you can do this earlier, but I did it when I had access to the second island and did it while attacking the Harmanse Gas and Repair outpost. Before you head here, make sure you have the marksman scope for your bow (so you can follow my reference point easier) and
SAVE YOUR GAME. Just on the outskirts of the outpost is a massive hill with a relic on it, located here:
Go up to the top of this hill (and grab the relic to get closer to the 60 you need for the achievement), then position yourself on the cliff looking towards the outpost. In front and to the left, you should see a sniper in a sniper tower.
Now equip the explosive arrows and aim at the tower. Aim straight at the tower then look up until the 100m line on the scope is lined up with the very top of the mountain to your right. Shoot the arrow and it should kill him, netting your achievement. If you miss, just adjust accordingly and keep shooting until you run out of arrows. When you do, just reload your save and try again. If you've already done all the outposts, you can reset them from the main menu.