>>719070485
I've done two runs, Normal, and NG+ And I have not fought him or demon of obsession.
First, because I did it blind, and missed four bosses in total. On NG+ I wanted to clear anything I missed, but didn't read carefully enough, and ended the game early with Bo inheritor.
But for me, Mistress Yan was by far the hardest boss. Nobody else came even close.