Gender: Unspecified Location: The balance of the Force
How many times must it be said that Post-Nib Sephiroth can only be banished by Omnislash? Nomura has alluded to it once himself, it is echoed again in KH2 and is stated in Ultimecia.
In other words the amount of damage you can do to him is irrelevant because he isn't killed conventionally.
The real question is: Can Sephiroth kill Thanos?
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.
Gender: Unspecified Location: The balance of the Force
Probably the same thing that happens when Genesis created a small sun around Sephiroth, didn't even burn his coat.
In all seriousness I think that is an issue FFVII has in multi-verse fights. Within itself it all makes sense but throw Sephiroth whom apparently is a god in all about reality warping against say Thanos and it makes none at all. Sephiroth is supposed to be all-powerful, and the guy who makes Chaos Vincent look like a pleb. However between game mechanics and Nomura insisting that he never go all-out in his appearances shit gets confusing.
In fairness if KH2 and Dissidia was taken seriously he himself might measure up better. Going by FFVII appearances only all we know is: He's better than everyone else by a stupid margin, he is a god and he can only 'die' via Omnislash.
TLDR: He is infuriatingly inconsistent on-screen compared to his Word of God accolades.
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.
I'm gonna need a source on that Sephiroth quote, as far as I'm aware Nomura has only said he's the strongest in the FF VII, the Ultimania Guides etc say nothing about Omnislash being the only thing to seal him, it's just a powerful physical attached k there is nothing super special about it.
And even then it's a moot point. Thanos can duke it out with galaxy busters, Sephiroth can't even destroy a planet on his own. This is spite any way you slice it.