An easy place to get this is on "
Agua Verde Charlie" fast travel point. You'll also need an accurate Sniper Rifle will long range as well as a powerful sniper scope. I used the M40A5 with the T5Xi Tactical Scope.
As soon as you spawn, you'll notice a sniper tower with an enemy directly in front of you at roughly 400m. Take into account the bullet drop and try to kill them. If you fail, you can continue to try to hit them or fast travel to the same location to respawn the enemies on the island in front of you.
You can also take a boat to this island and kill the sniper with a pistol to get: