This page of the guide to Resident Evil Village features a description of the boss fight with the third and last daughter of Lady Dimitrescu in the castle Library. The following tutorial explains how to avoid the vampire's attacks, how to temporarily weaken her and how to ultimately defeat this boss.
The clash with Lady Dimitrescu's final daughter will begin when you reach the Library on floor 2F of the castle. Ethan will arrive at this location right after solving the piano puzzle in the Opera Hall, getting the Iron Insignia Key, and escaping Lady Dimitrescu, who will appear in the piano area.
Just like in the previous two fights, you need to defeat this vampire with a trick. Examine the central area of the library. Ethan can find a lever here.
Pull the lever when the vampire is standing in the middle area, as this will open the glass roof, through which icy air will begin to enter the library. As you remember from previous fights, Lady Dimitrescu's daughters are susceptible to the cold. Use this fact and begin your attack.
Unfortunately, the last daughter is smart enough to close the roof every so often. In such case, you must stop attacking her for a moment, go to the middle of the room and wait for the vampire to reappear. As soon as this happens, quickly run to the lever to open the roof again.
You can find ammunition in the library - a shotgun will work best in defeating this daughter. Just like the previous sisters, you have to make the boss turn into stone and fall apart. The third vampire will leave the Crystal Torso treasure behind.