She is very strong, aggressive and with the ability to heal herself - pay as much attention to her as you pay to the boss fight. Bloodborne Assassin Woman in white. The best way to start is to wait under the stairs until she will stop still in front of the chest on the second floor. There are strange items in that chest - student clothes - is that so valuable to guard and protect? :) Once she stopped walking - backstab her - it will give you a good start for your fight.
