Bloodborne: The Old Hunter Lady Maria of the Astral Clocktower boss battle guide to help you acquire the Celestial Dial as a reward.
The entire fight is divided into three phases during which Maria gain increased damage output and effective range:
After the battle starts, make sure to dodge in forward – left or right – so that you are in a position to counterattack.
If you continue to hit her, you will probably stun her, but make sure to avoid her incoming attack if you see her back away from you. You should be easily be able to tell her attacks without any issues.
As soon as she drops down to 50% HP, she will stab herself followed by a devastating AOE attack. Make sure to back away from her in time to avoid damage.
This also triggers the second phase of the battle in which she acquires new attacks and gains pretty long range. In this phase, she will easily attack you even if you are behind her.
The good thing that you need to note here is that you will still be able to stun-lock her with most of the weapons at your disposal. When you see her getting her weapons behind her, make sure to get behind her as she will follow you.
After dropping down to 25% HP, she will lunge into air triggering the third phase during which she gains even more range as well as damage, but all of her attacks and tactics remain the same.