Supernova isn't a canon move. It can't blow up the Sun. It can't harm the solar system. It can't even kill teh party. It can't kill Alucard.
And no, Kain wouldn't die like a normal person. He'd warp dimension, freeze time and get a free-hit on Sephiroth, effectively obliterating him. Hell, Kain could take on the Justice League and stand a chance. He simply has too much power magic spells to rip out souls, a sword nearly cosmic-level in power with control over dimensiont, ime, elements and not to mention his intelligence. He beats the best of the best with ease.
Even without his immortality, he'd kill Seph's ass.