User blog comment:MiraMajin/Elfman's True Power/@comment-24536109-20140225112007

Actually we don't exactly know what happened, Mira might have held back considering its Elfman of all people.

If anything someone did stop Elfman from rampaging considering he still can't control it and only managed to control it later in the series when Elfman fought Sol, and Mira is the only viable option that time that could have stop him. So if anything Mira having no choice left after seeing Lisanna "supposedly dead" beat the Beast.