Complex sentence: She alone of the First went up against a Sigil-Bearer, Giselle Villard of "Mystic", and defeated her, but did not kill her; indeed, she helped her by removing Animora's hold over Giselle.
Split into simpler sentences: She alone of the First went up against a Sigil-Bearer and defeated her (this was Giselle Villard of "Mystic"). But she did not kill Giselle, indeed, she helped her by removing Animora's hold over Giselle.