The easiest way to guarantee a single enemy where you can land directly from a bivouac is from the Sunken Clipper Bay Bivouac in the North side of Smugglers Cove. North from the bivouac there is the Maunga Nui Port.
Within the port is a large building with a single sniper on its rooftop, usually looking north, with his back to your jump point.
As long as you have purchased the parachute skill you can base jump by holding
as you run off the cliff facing the port. Have some faith and take the leap, land on the roof and kill the sniper within 5 seconds and pop goes the Achievement.