I haven't explored the whole map after completing the game, but I'm pretty sure this is the fastest "circuit" for obtaining this achievement. Each "lap" grants you 3 enemy encounters (2 if some enemies fail to respawn) in a couple of minutes.
(1) Fast travel to "Logging Camp" Base Camp
, in Soviet Installation
(2) Go to the central structure. There are 4 enemies
camping in the right side of the building.
(3) There are 2 enemies
patrolling around the rocks southeast of the first group of enemies (they may be northeast sometimes). They may attack you while you are fighting the first group, but they'll still count as a separate encounter.
(4) Go to the Copper Mills entrance, where you'll find 4 more enemies
(5) Go to "Copper Mills Yard" Base Camp
and fast travel to "Logging Camp" again. Repeat the process.
Sometimes, the third group of enemies doesn't spawn, but it will be there in your next "lap". With this method, you'll obtain the achievement in around 10 minutes.