Weird West: Definitive Edition

Random encounter with the Heathen found on Chapter 4 as the Werewolf. She'll challenge you to slap her but you'll fail every time unless you have the "Amulet of the Open Palm". The Amulet can be found in the temple of "Somnolence" in the south eastern desert. Fight your way through the temple into the main office and you'll see the legendary Amulet sat on the desk. Now that you have the amulet you can fast travel around until the Heathen appears again. Make sure you have the Amulet equip before speaking to her! Now you should have a fifth option to use the Amulet of the Open Palm to beat her. Once slapped the achievement should pop.

During one of your encounters with the Heathen as the Werewolf she will challenge you to a slap game but you will not be able to win without a certain item. You can get this item from the Oneirist temple in the desert called Somnolence either by going there early as the Werewolf or waiting until the very beginning of the Oneirist Journey where you'll be tasked with entering the room to steal a stone doll. The item in question is a gold amulet that can be found on top of the desk in the far corner room which you can enter by opening the grate in the water.

*EDIT: It seems some players are reporting that completing this early during the Werewolf journey may prevent a certain encounter with the Heathen that is required for another achievement, so to be on the safe side it may be best to do this during the Oneirist journey instead.