If you collected the bastille key you can open the two gates to the sides of the Lost Sinners fog door, light the trail of oil inside each room with a torch and it will light up the boss room, extending lock on range. If you're human you can summon Lucatiel of Mirrah (at the bottom of the elevator that takes you to the flooded dungeon) and you can also summon Sellsword Luet (his summon sign is in one of the cells to the right on the second level of the flooded dungeon). They can tank most of the damage from the Sinner for you, specifically Luet since he wears a ring that makes enemies target him more easily.