Tips on dealing with a spammy Braixen as S. Mewtwo in Duel Phase.

I've run into a couple very spammy Braixens today. I did pretty good in Field Phase, save for a few mishaps with Braixen's small fireballs and the rainbow shot at the end.

Duel Phase was a nightmare though. Once they got me away, they start spamming the giant fireball that takes the whole screen once it grows. (I don't know the name.) And to my experience S. Mewtwo can't seem to take it well. Am I just doing something wrong?