Thank you for checking out our entry and for the feedback.
I based the final boss and their 3D6 on the 'enormous monstrosities' from the core rule book but I think your right that the AoE element of the attack makes that very OP.
I've just pushed an update to reduce this down to 1d6+1 to all players.
https://atlasarcane.itch.io/fist-afterlife/devlog/391339/final-boss-encounter-up...