You'll likely get this through natural play, but if you'd like to get it quick, the first level you can do this on is level 1-3.
*Note: The later levels will have more grouped enemies, and you'll likely need to use airstrike more often anyway, so I recommend saving this during natural play so you're not wasting command points.*
This is also a good level to get:
Start up level 1-3 on easy (one bullet). Firstly, order some airstrike from the yellow "C" in the upper left. It costs 18,000 command points. Wait for it to drop in, and you're ready. From where you spawn, go up, to the right, up again, kill the three enemies behind the wall. Then go left. You should be near a tower that is shooting at you, and a bunker. Try to avoid the tower's shots until four enemies come out of the bunker. Then, move back to the left, where you came from, and use air strike. You'll want to use air strike in the direction they're walking, not directly on them, since it takes a while for the air strike to be deployed. If you killed at least three of the four, you'll have your achievement.