You'll get this over time as you're playing multiplayer, blitz, and skirmish, but if you want to boost it you just need to use these units against enemy air units.
Also, if you're boosting with a friend in a private match, Strongholds where you leave one Stronghold uncaptured goes into Sudden Death and you have lots of time to do these achievements for specific kills.
Sometimes if any other type of unit happens to hit your target, then it seems to cancel out the 'kill' for the purposes of this achievement. Sometimes it seems like you have to get the kill 100% with the anti-air unit.UNSC:
Additional UNSC units that might count for this achievement because they are good against air units:
Additional Banished unit that might count for this achievement because it is good against air units: