First thing to say, there is no technical issue in earning this achievement from WoL Ghost of a Chance mission.
Take Reapers and kill two Spectres with them and you can get the achievement without any trouble. My question is how about the kills using G-4 Cluster Bombs ability - the Hyperion Armory tech available for Reapers - instead of ‘normal’ ground unit attacks. I tried, but it doesn’t work.
Probable reason of this is because these bombs are classed as individual units - similar to Spider Mines - so Reapers are not credited with kills even though they are the very casters spawning them.
Perhaps SC2 dev team may have intended all players to earn this achievement with just a normal attack, after all. However, with overall integrity of these 10th anniversary achievements being questioned as many of them came out with bunch of bugs needed fixing, I can’t let go of the feeling that it may be another oversight from our team as well.
So while fixing recent achievement bugs - typos and imperfect fixes still being addressed - I hope the dev team looks into this and see if any changes are needed to be made. Thanks.