Seeing enemies using energy shields and shooting at you with plasma pistols? That is a Jackal, that or being sniped from a distance is a jackal as well.
See either of these two, kill them and they will go towards this achievement. 100 of them will unlock it. Difficulty does not matter.
I found a very educational video for to watch and learn about jackals, and possibly a good laugh, by Teh Spearhead. (The animation on the cover is not the animation through-out the video)