Weevil

If I remember correctly, you need to use the Mirror Shield to aim the fire/ice beam at the opposite sister. You first need to lock-on to one of the sisters, then cancel the lock-on, so that you can direct the beam towards the other sister (use the joystick).
The second part is where you need to charge up the Mirror Shield with 3 of the same magic (fire/ice), then direct it back at the boss. If the boss launches a magic attack that's the opposite of what's stored in your shield, move towards the edge to avoid getting hit.