Can anyone share any advice on how to earn this achievement?
The description says: Fight a legendary monster in single combat.
I’ve played through the game over 20 times and earned every other achievement, but I have no clue how to get this one. There is no mention in the story about any ‘legendary monster’, nor have I encountered any situation where I could potentially encounter such a monster.
You have already met the “monster” during the main path. It’s not some obscure not-yet-seen enemy that has some kind of a super special unlock condition.
You need to have a decently high combat stat to trigger the duel event. It happens in Chapter 5.
You also need to be carrying a steel sword to win (don’t offer it as tribute to the turnstile/demon).
The “legendary monster” is the goddamn janitor robot.