This should be fun (should I do one where they actually have equal support?

Doom Slayer is basically a figurehead for a Divinity at multiversal strength

Ok, but Sephiroth can literally summon meteors if he wanted to, which would render this battle completely useless. Also Sephiroth has materia and can use giga flare and other magic.

The right. The entire squad. It kills the left within a nanosecond

Tbh I honestly think Sephiroth has a chance at winning, he did kill the equivalent of God in SSBU in a nanosecond just to traumatize Cloud some more. Personally, I say the right side wins but not without some serious casualties.