I got this after defeating Achilles. The fight didn’t give me too much trouble, I just kept dodging his attacks and hitting him with my axe.
The Wraith of Achilles popped up for me after I beat him while helping Aphrodite (she was the first god(dess) I helped). I believe the wraith has the spawn in the region to be defeated.
If you downvote, please let me know what I can do to fix the solution!
EDIT : As the comments have stated, you have to defeat the wraith in their lair for the achievement to pop. You fight them in the open world to unlock their lair.