Hit an airborne enemy with the Shotgun's Slug Shot.

You acquire the Shotgun during the story mission 'Blackout' when asked to 'Investigate the Ark'.

This achievement can be finicky as the wording seems to be ambiguous. You need to send an enemy airborne yourself, not simply hit an enemy in the air. If an enemy flies under their own power, hitting them with a Slug Shot will not unlock this achievement.

There are a few ways to achieve this, but the best method's are using either 'Slam' or 'Vortex'. Using 'Slam' you need to make sure you do not have it fully leveled up as the damage it does will likely kill enemies outright, especially on easier difficulties, however on higher difficulties it may still be viable when fully upgraded. 'Vortex' can be fully upgraded and remain viable at any difficulty, even doing so may make this achievement easier, though it is not necessary. To use the shotgun's Slug Shot you need to simply be aiming down the sights when firing.

What you will want to do is equip your shotgun and have it ready pointed in the correct direction, then either use 'Slam' ('LB' + 'A') to send your chosen enemy flying back, or use 'Vortex' ('LB' + 'Y') to have them float in the air in front of you, then quickly aim down your shotgun's sight and shoot them. 'Slam' is much harder as the enemies hit will fly backwards at a decent speed and you will need to readjust your aim to hit them, whereas with 'Vortex' they will simply float in front of you for a second or so giving you much more time to line up a shot.