For this achievement you must complete a level without killing any enemies and if you played the entire game without killing all the enemies you'll also get the Pacifist achievement at the end of the game. If you didn't complete a pacifist run the first time you can chapter select and replay the chapter. the same also applies for the Yurei (no alrets) and Oni (kill all enemies) medals.
* Edit thanks to Timww572 difficulty doesn't matter for. Timww572 played the first chapter on medium difficulty and then got everything else on easy and unlocked the achievement
I should also mention the last level of the game chapter 13 you get all three medals Kami, Yurei, and Oni after defeating the final boss and credits roll. You'll get the respected achievements for getting your first Yurei and Oni medal if you didn't earn them the medals throughout the game