[WaifuApple]

The mirror knight boss seems more likely to come in a separate dungeon with the wasps and beetles that also currently do not appear in any dungeon, as far as I am aware. Rather than just throwing it where there's no point in it being, I'd advocate for some sort of temple BDP with this as the boss. One or two more new mobs to go alongside the two I'm aware of also from Bai Kai and you'd be set.