Warframe

Mite Raknoids count for this, they're the little green ones that come out of the ground when you're farming minerals just to piss you off. Just kill them while being in the air when they spawn. They spawn in groups of three so you'll need to get four groups to get the achievement. They also seem to be random about spawning so I was just doing a lap around the starting area pond with the grinding pipes and killing them if/when I found them. Make sure you don't have a companion/sentinel with you to avoid them killing the Raknoids.

This only took me about 10-15 minutes.

I initially tried the method of running around the entry to Fortuna but i had no luck so i just ran around the snowy areas on foot. So equip a weapon that has a bit of crowd control, for example, i used my ignis. Also unequip any sentinels and just run around in the snowy areas. When you see a group of raknoids, simply double jump and hold left trigger to aim glide then shoot them. Equipping a mod that shows enemy locations on your minimap might also help but not necessary.

I tried to run around like suggested by the other guides. Near Fortuna entrance, close to caves, below the giant mushrooms. Nothing at all. Wasted 1 hour and got 0 kills.
Then I found an EASY and CERTAIN way to found the Mite Raknoids (the small green ones).
Go to Fortuna and pick up the first mission (Phase 1) of the Profit-Taker Orb. You will have access to such mission once you'll reach the highest level with Solaris United. If you don't know where to find it, go in the backdoor behind Eudico. There's a room with a table, Eudico and Business around it and Little Duck in the end of the room. Talk to Eudico in this room or interact with the table.
During the first step of the mission called "Phase 1", you will be asked to fish in a certain cave. While doing that, a random number (3 to 6) of Mite Raknoids ALWAYS SPAWN, directly attacking you. Little Duck will inform you of the "danger". Simply seize the opportunity: stop fishing and kill the raknoids while airbone (bullet jump, then aim).
With 2-3 repetition of this mission, you'll have your achievement unlocked. Moreover, you can also get interesting rewards completing the mission. Anyway, you can also leave as soon as killed the Raknoids to speed up the process. It takes 10 minutes doing this way to have your achievement popping starting from 0.
Tip: don't bring the sentinel with you, play solo, use a weapon with a decent damage (the Raknoids here are more or less level 50) and possibly with an area of effect like Ignis, Arca Plasmor, Lenz, etc.

To get Airborne Exterminator achievements you must kill 10 Raknoids while airborne.
I made a video, you can see how does they look like. Place is near enter to Fortuna.