HIGHLY missable achievement - you can only unlock this by killing the spirit. If you let it live, you won't get the achievement. Luckily, if you've already completed it the way that doesn't unlock the achievement, II RedDwarf II found a great workaround.
Start a new game on Heart of Stone DLC to start on level 33. You'll be dropped into Velen, immediately able to fast travel to Fayrlund, Skellige (the unmarked town just below Rannvaig), go to the Quest board, and start the mission from scratch. Quick and easy from there, just make sure to kill the Spirit this time. Done!