To unlock the Fair Prey Achievement in Little Nightmares 2 you'll need to equip the Raccoon tail hat that can be found in the hunters house during chapter 1 and make sure you're wearing it when you use the shotgun to kill the hunter. Here is my own video guide that shows how to get the Fair Prey achievement. The raccoon tail hat locations is shown at around the 1:38 mark during the video and even though the kill on the hunter is unmissable it is shown at the 5:52 mark in the video.