Cloaked Hunter enemies are first encountered in Gideon’s Rock area. They are not very hard to kill but they can melt you down pretty quickly if you are not careful.
For this particular achievement you have to kill a hunter who is cloaked. It will try to cloak in the middle of the combat. Try weaken it’s health that it has some little of health left before it cloaks. Then let it cloak and dont interrupt it. While they are cloaked just swing your weapon (preferably spear or hammer) here and there and you will most likely hit that cloaked hunter and kill him straight away and get you're achievement along the way.
Personal note: Cloaked hunters usually will try to attack from sides.