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.