Kill an Authority Enforcer during Jetpack descent
Easy to unlock during the "Liberate Captain Marshall" mission. While manning the turret, try to kill one of the enforcers as he descends using his jetpacks.The easiest way to do this is to hit them in mid-air with any hitscan weapon. The enforcers hover a few meters over the ground before eventually dropping down. Take this opportunity to blow an enforcer out of the sky. If you missed it in "Liberate Captain Marshall", you can try again during the "Assault the Authority Bridge" mission.