How do I defeat the witch?

Hint 1 of 4:

First, you'll need protection from being turned into a frog.  See the
previous question if you don't have that protection.

Hint 2 of 4:

Have you been into the thieves' treasure room (in the desert) yet?

Hint 3 of 4:

You must have the brass vase from the treasure room.

Hint 4 of 4:

Have you opened the vase yourself?  (Save your game first!)