First, defeat tribal enemies until you acquire a tribal mask. Equip it to move among them undetected. Along your travels, you will sometimes randomly encounter a tribal chieftain sitting on a rock. Talking to him will lead to a variety of scenarios. If you are wearing the mask, you will have the option to flirt with the chieftain. Succeed, and he will take off his mask for you.
They may be other ways to unlock this achievement.