Another trick to beating Ethel during the battle near the cage is to hit her with any debuff type spell (bane, hex, hunters mark...). After she creates her 'clones.' only the real Auntie Ethel will be marked with the condition you hit her with. Then, as JustAnotherBaldu said, hit the clones with any attack (magic missiles are perfect) and they disappear, hit the real one with the hard attacks to kill her.
