It's a good problem to have though.
Now keep this in mind:
Mega Mawile has the ability 'Huge Power'.
'Huge Power' DOUBLES it's attack stat.
If it keeps it's ORIGINAL stats when Mawile transforms into Mega Mawile. that would double it's base 85 Attack to base 170 Attack.
Right now, the ONLY Pokémon with higher base attack than that is 'Attack form' Deoxys.
That would mean this little Nu Fairy/Steel type would then have the second highest attack in the ENTIRE game!
Or, maybe it's like a limit break where if you fall below 25% it activates and you transform.
We still don't know.