Summoning Jester Thomas makes this encounter ridiculously easy. You can actually just hold your shield up or kite and let the Thomas kill her.
Check out the same fight without the summon: http://youtu.be/CFKZ-so1KME
"Mytha was formerly engaged to the Iron King of Alonne. Unfortunately, the king had no affection for her in spite of her beauty and fell in love with another. The knowledge of this drove her mad, causing her to poison herself in the hopes that she would attain further beauty. Instead the poison only corrupted her and transformed her into a monster. The king then banished her to the Earthen Peak. A man that is believed to have loved her followed alongside, however, as she was oblivious to the affections of any other man that was not the king, his feelings were ignored. That man is said to have transformed into the monster known as the Covetous Demon."
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Mytha, the Baneful Queen is a boss in Dark Souls II. Located at the end of Earthen Peak.