Golgonar's Comments

looks like this achievement is meant to be timegated. As i understand you would usually have to wait for this quest to come up 3 times to get 6 out of 18 each time. However when you are in a raid you can get all 18 in one go without them despawning on you once you have 6 for the wq. 

Edit: Looks like the moonkins do disappear once you saved 6, at least i cant find any after doing the wq but you can cheese this as i mentioned.

Just as Wcins1318's comment suggests i got the scale on the very first kill on kob'rok after finishing the tablet, another player in my group got it too and got the achievement

Dunno if this makes a difference at all but since there is limited information on how to obtain this pet i exclusively trapped souls from the Blazing Inferno in Thaldraszus. Figured it makes sense that i get Blaze Spirit from the Blazing Inferno. Got it after opening 10 Docile Fiery Souls.