Though definitely one of the strangest bosses in Bloodborne, The One Reborn is thankfully one of the easiest to handle. The first thing you are going to want to make sure and do is run up the staircase, to the back and right of where you first enter, and kill those pesky Bell Maidens. If not dealt with, they will cast fireballs and an assortment of other nasty things at you. Once they are dealt with it’s time to deal with The One Reborn. Hit him in the midsection while keeping a close eye on the man at the top of the flesh heap and dodge whatever spell he cast. Keep this up and put these poor souls to rest.
Bloodborne Bosses: The One Reborn Strategy