I think I know what I'm going to do.
The players will go to get a powerful ring from Pelor that will channel his holy light to weaken the frostfell queen. They will also go to where she drained the god's power to claim an amulet she created to defend herself from the god's wrath, which in turn they can use to defend againt her attacks. They can also contact the vestige of the god she stole power from, who will give them his blessing, which will help protect them from her necrotic spells and abilities. If the party has learned her true name, which they can do if they perform some side objectives throughout the campaign, they can use it to remind her of the time she was a powerless orphan, which would weaken her further. If the PCs think of it, they can also lure her to a location like a holy site for Pelor to weaken her further as well.
How does that sound?