Well, I don't know much about Magus considering that I HATEChrono. So I'll list some of Sephiroth's attacks.

Ice3, Fire3, Bolt3, Massamune slash, Demi3, Bizzaro Energy, Stigma, Pale Horse, Wall, Black Wing Slash, Supernova

Well NO ONE can dodge Supernova. It could burn the shit out of Magus. Considering that Sephiroth has "All" materia attatched to his level 3 spells, Magus couldn't dodge that either. He could cast Wall to block Dark Matter.

They're both cool-headed from what I've observed. They aren't hot-headied or stupid. They would observe their foes. Well, Sephiroth goes slightly insane when he becomes evil, so he won't be as cool.

He could cast Bizzoro Energy to keep himself alive. Then with his speed, he could attack with Massamune Slash.

Battle:

Sephiroth(Human)
Massamune Slash. Fire3.
Magus: Dark Matter
Sephiroth morphs into Bizzoro Sephiroth.
Bizzoro Sephiroth: Bizzaro Energy. Physical attack. Demi3. Then he could do Stigma.

I don't know all that much about Magus, but that is a good enough ammount I think.

Science Note: Our sun which Sephiroths destroys is not a large star. There for it can not Supernova. Only large stores after become a Red Giant can explode ending in either a black hole or that really dense thing.