I loaded up adventure mode for a new roll on Yaesha, the Forgotten Grove has to be the starting area for the ravager storyline. I then set my game to Offline mode.
To turn on Offline mode. go to the system menu. From here, set the game to Offline mode.
When you get Yaesha, play through the world and if you’ve got the right roll you’ll reach the Forgotten Grove area.
After you solve the Water Harp Puzzle you can meet the Ravager. Once you do, talk to him and he’ll ask you a favour: to kill the doe sitting in front of him.
Grant his request and you’ll unlock the achievement
Defeat a Boss in Single-Player Without Taking Any Damage