To unlock the Merciful feat achievement in Little Nightmares 2 you'll need to get around the bully that is tied up in chapter 2 (The School Chapter) without killing him. To do this you'll want to stand on the edge of his reach and when he pounces for you he will pulled back by the rope, at this moment run for the pipe, pick it up and use it to smash through the broken door on the other side of the room. Here is my own video guide that shows how to get the Merciful Feat achievement along with all of the other collectibles/achievements that Little Nightmares 2 has to offer. The Merciful Feat achievement guide is shown at around the 9:31 mark during the video.