Knuckle Sandwich Achievement

You can get this achievement in either the Werewolf or Oneirist chapters. Once you are playing as one of those characters, you will want to make your way to Somnolence, a temple in the SE section of the map. In the final area here you can find the Amulet of the Open Palm, an equippable item.

During your travels, you may get a random encounter (as always called A Witch at the Crossroads) where you will be challenged by The Heathen to a slap game. The only way to win this game is to have that amulet equipped prior to speaking to her. This will give you a new option during the game to use the amulet, allowing you to win the slap game and unlock the achievement.

Don't worry if you get this event prior to finding the amulet and lose, you can trigger it again and win later.