The person to ultimately beat Mithos' final form is Soifon. She uses her Bankai to do it.
Mithos' decision to make a Heel-Face Turn. After he is revived by Orihime he is indifferent about Lloyd and the others' attempts saying that he simply won't interefere. However, after he bears witness to Lloyd's undying will with fresh eyes he changes his mind and summons Maxwell to help Lloyd and Colette revive the Giant Kharlan tree.