You can do this about 5 minutes into the game if you want to get it out of the way. It is the second group of enemies among some catwalks. If you get noticed or even if a dead body gets noticed, they will sound the alarm and a chopper will come. Equip your bow and put on either thermite or airburst arrows and fire away. It should only take a single shot, maybe 2. Took me 1 shoot on super-soldier with airburst.