Bionic Commando

When you unlock the Death From Above move, you can perform it by repeatedly tapping the Y button while falling from a sufficient height (it will prompt you on the right side of the screen if you're falling from high enough). When you hit the ground you'll deliver a small shockwave that will kill simple enemies in a single blast.

Once you kill your first enemy in this fashion, the Achievement will unlock. The first opportunity you are likely to get is shortly after earning the ability when the paratroopers arrive after you hack the nearby relay. Return through the sewer pipe, and when you jump back up to higher ground, use the nearby signage to launch yourself upward with the bionic arm, and aim for a grunt while tapping on the Y button as you fall back down.