There is a great spot in the game with 6(!) enemies standing together. This opportunity can be found later on in the "Research Base" level when you are given the task to
Stay to the left of the map as much as possible without alerting the enemies until you discover a new base camp. Swim underneath the ice, and drown the first enemy standing next to the opening using a water finisher. Get out of the water, and kill the second guy. Immediately
turn around, and shoot a poison arrow in the large group of enemies standing next to a red truck. *Achievement unlocked!*
You can restart the checkpoint at the base camp as many times as you need.
Here is a video of me getting the achievement: