It would be something like, the Crimson Moon dropping a mirror image of the Moon (HERE IT COMES!) versus Zelretch doing something only he was able to do: a virtually limitless Ether Cannon unleashed through a special magic circle ("Magic Square"). If we had five times the budget of Mahoyo, then we may be able to realize such a scene.

TYPE-MOON Fes. Official Pamphlet 10th Anniversary Q & A Booklet:
Q: You must tell us the details of the battle between the Crimson Moon and Zelretch at his prime!