This boss is not located in the Orphanage. It's an understandable mistake, especially from a progression standpoint. But, the Orphanage is actually the building that serves as the main area of Upper Cathedral Ward. The Orphanage Key opens the front door to that building, after all. If you read the item description of the Orphanage Key, it tells that it was a place of education and research, where orphans were raised to be scholars and researchers. So this boss takes place in the Lumenflower Gardens, which are not to be confused with the Orphanage (or, for that matter, with the Lumenwood Gardens).