How am I supposed to earn them if I don’t even know what they expect me to do for them? Why do these exist? I haven’t even encountered a game (not even a Fromsoft game, and those guys are notorious for being vague and not forthcoming with info even for guide books) where the hidden achievements were not already listed by a 3rd party somewhere outside of the game day 1, so why not just show them in the damn game?
You can be vague enough that it doesn’t even tell you it’s a secret boss. Or make it part of a larger achievement like “defeat all bosses.”