You do not have to be stronger than someone to beat him.
Strength/Power exists in many different forms.

So that shows no matter how strong the characters are they are not stronger than him.
Individually, no. Except for maybe Cloud.

Because it is not necessary. Sephiroth has his arrogance weakness.
Cloud's determination and will IS a form of his strength and it's what allowed him to win in Advent Children.
This is just your opinion and can't be really proven. Like my explanation about why Super Nova is a power demonstration cannot be 100 % proven without Square as well.
*facemoltenlava*

oh my god not this again

Alright, let me explain how it works. You brought up the assertion that Sephiroth can use Super Nova and you need to provide proof first as to why your statement is true. You have to do more than say "well it's a possibility with nothing to prove against it." If you can't provide concrete evidence that he can in fact use Super Nova, then I don't need to provide proof as to why the assertion is wrong because it automatically fails on its own.

Here's an example that I like using to explain why this commonly used debate tactic doesn't work:

Child A says that unicorns are real.
Child B says "you can't prove that unicorns are real"
Child A says "that's just your opinion and you can't prove me wrong"

In this case, we all know that Child A's assertion that [unicorns are real] is false due to the fact that he couldn't provide his own proof. Child B didn't need to provide his own proof against Child A if he couldn't provide his own proof in the first place.
It doesn't explain why Super Nova should be the only move in Final Fantasy who doesn't show what a character can do.
I'm saying that he couldn't use Super Nova because you haven't provided evidence that he could except for one question that wasn't related to the plot.
it is fact Sephiroth can use a move called "Super Nova" - why should it look like a Super Nova if he cannot unleash a real Super Nova? This makes no sense. He doesn't have to impress other people. People already fear his power.
...it's just a name that Square Enix made to sound cool, dude. It doesn't have to do with anything that the actual attack does.