Shoot an enemy in the head. Aside from random headshots, if you want to take the time, use the R stick to aim at the head of a soldier and fire. 1 shot, 1 kill. When you get a successful headshot, a comic book style white blotch of exclamation will appear above the soldier's head for a split second.
Note that with the Inertial Element, the final gun, it is harder to get headshots than the others.