A webcomic that I follow recently showed a Magical Girl whose primary magical ability was super-speed [She had a weapon as well]. Thing is, by herself that is all she had and it wasn't enough to keep her from losing an arm and being depowered.
You would need a Magical Girl who had a powerset that included super speed, as well as other abilities including durability. Of course, if you had someone with near reality-altering magic that could disable anyone else regardless of how powerful they are, then maybe that might work. But even if Wanda, the Scarlet Witch went up against them, there is no certainty that she'd have the time to do anything before being knocked unconscious.