I'm not very aware of the full breadth of options as well. If you have Athletics it might be worth trying to Trip or Grapple. Physical attacks barely seem to dent it,so magical attacks might be a good idea but we might risk damaging Rhaemy. If any of your cantrips is a buff,that might be a better idea to assist her escape attempt?
Can you directly heal her in case she takes more damage trying to free herself?
Another wonder I have is if we're intended to stop the cauldron by another means, like taking something from it's master's remains or taking some other action in the cabin.