|Grab a sniper from a guard in a sniper tower, (you can also buy a supply drop from China, PLAV or the Pirates for one) Run into a hostile area and wait on the road for jeeps with turrets to ambush you. Once they start coming, snipe the driver once he parks to let the turret gunner shoot you. Make sure they stop early so they don't run you over. Move out of the way if they don't. If you kill the turret gunner first the driver will get out of the jeep.|
Be sure not to waste bullets. Reloading wastes a whole clip, so let the game reload for you. I recommend going North East of the map. Where China and the AN are fighting. There is tons of jeeps that spawn for china. You can also go around the HQ of UP. Tons of their regular pickups with not to many tanks.
You can also go "West of the PLAV HQ is a good stretch of road that spawns VZ jeeps if you run from the barracade to the bridge and you can avoid dealing with random Hostile backup" (thanks Badmojo)
If you don't unlock it by the time the game is over. Get a sniper with tons of ammo and save the game. Go out and snip the drivers and quit out. It will save progress towards the achievement but keep you at max ammo. Be sure not to pick anything up with the chopper, as it will auto save.
It might take more then 50 kills to unlock so just keep going at it.