Another easy spot that I used to unlock this achievement.
In the Western Precinct, there is an airstrip in the North just above the
Agua Verde Brovo fast travel location. Without a helicopter its a steep
hill climb up the stream.
A friend and I were going for the (Extranjeros R4 weapon) in this location
and found 3 regular enemies and 4 Covert enemies. We took turns killing.
If you kill yourselves in this location it re-spawns you right with these enemies.
If your solo good luck dying without getting revived.