The previous guide on conflict zones is excellent and is probably easier to get up front. But if you are coming into combat after you have built up some money in the other pathways then this is a way to stack several combat achievement-getting runs together.
As of the time I'm writing this solution, hunting Thargoids (including Scouts) means that the Pilot's Federation will give you combat bonds that you can cash in, even if you fight them at random Non-Human Signal Sources instead of in AX Conflict Zones. So if you are going for the Thargoid-hunting achievements then you will get these naturally. (along with the Elite ship combat achievements; if I understand correctly, the Thargoid Scout combat rank grinding strategy recently got nerfed so they now only count as Dangerous, experience-wise, but they still seem to count towards the Elite...Dangerous and Elite...Dangerous-er achievements).
I used the Krait Mk II build presented excellently in 2019 by Down to Earth Astronomy on YouTube:
I did not engineer my Krait whatsoever, so it isn't necessary to do the engineer grind before going hunting for Scouts, but it will obviously make you a lot more resilient if you do so. Then I jumped over to Asterope in the Pleiades Nebula (check the Anti-Xeno Initative for their meta on the best Thargoid-hunting system at the present time:
https://www.antixenoinitiative.com/wiki/thargoids/finding-th...). I personally found it easier to jump in the Asp Explorer I kitted out for long-range exploration (see the Exploration section of the Walkthrough for the build), then shipping the Krait to Copernicus Observatory in Asterope and just waiting the hour for it to arrive.
Once you are at the target system, look for Non-Human signal sources of threat level 4 (which generally only have Scouts) and you are off to the races. With the build linked above, so long as you can keep up with the Scout you are targeting and you don't get yourself surrounded by too many of them at once, they are quite spongy and go down very quickly. I had enough for this achievement after just 1 or 2 signal source sites, which I discovered, visited, and then mopped up in easily 5-10 minutes.