An air grab is done by jumping up with and then grabbing your enemy in mid-air with . You can use this move when fighting enemies with machetes as they will often lunge towards you. Note that lunging towards them and throwing them away or just grabbing them from the air does not work. You only execute an air grab with the described method. There is not really a place to grind this achievement, but if you wanna get it quick, do it when and right after fighting the first Machete Champion of the game in the first level.
Usually you will not instantly kill an enemy with an air grab, so you can use this attack on them multiple times.