*note* Although this achievement was once reported as glitched for a lot of people, it has been reportedly patched.
This is a very nice guide i found when i obtained this achievement. Using this i managed to get it in 20 minutes.
ALL CREDIT GOES TO Odwolda:
What you'll need:
-A good sniper rifle (I used the Anti-Material rifle from AN. I bolded it because people were asking if they HAD to use a sniper rifle and yes, you do), and STOCK UP on supply drops. Plan to use a lot of ammo, even if you don't think you will. That way you won't find yourself running back to get more ammo.
-A fast helicopter (I like UP's Rogues, any of them will do).
-The UP airport outpost unlocked (this is the one all the way left of Maracaibo, up on a hill).
-Hostile with UP. The achievement requires ENEMY drivers to be killed.
What to do:
-Fly over to the UP airport outpost, then go just a little south of it, however where you want to go is still inside the base. You'll see two parking garages, right next to each other. Land on one. If you land on the northern one, your targets will be north. If you land on the southern one, your targets will be south. However, targets CAN be all around you so always check around.
-You'll see UP guys driving Ravens around on the ground, and they won't notice you (until you shoot or aim at them for awhile). As required, start shooting drivers. 95% of the time, they'll drive and pull over and stop, giving you a nice shot. For each time you land (you'll see what I mean later), you can probably find around 4-6 Ravens driving around, which means 4-6 drivers every minute or two. If you run out of ammo, call in a supply drop. If Ewan misses (he tends to) and the box goes off the top of the garage, just get in your helicopter and use the winch to pick it up and take it to the top. This way you aren't at risk.
-When you think no more Ravens are spawning (you'll see a bunch of guys on the guns of driverless Ravens), get in your helicopter and fly over to the other garage, you might need to fly around a bit. This should "reset" them, and a new batch of trucks will appear. Simply repeat this process and you'll have this achievement in no time.
I never had anyone come up the garages or helicopters fly over to get me, so you should be fine. If the trucks get close enough the guys on the turrets can and will shoot you though, so just be careful. If you fall off the garage, you should only lose like 10 health so just run to the entrance and get back up to the top. NOTE: Killing the turret gunners doesn't count so just leave them be.