It's literally impossible to kill Final boss Omori. The only way to even just "defeat" him is by playing Duet, that also requires someone else to play a piano, specifically Mari.

First of all, this fight won't even take place because canonly, OMORI is Sunny's imagination, in his head. So Frisk won't even be able to fight him. And even if this fight somehow does take place, it has to take place in Whitespace, where OMORI has control.

Secondly, when this fight does take place, many people will start saying that Frisk's sheer determination will help overpowering Omori. I want to ask, how? What's the proof that Frisk's determination is powerful enough to easily defeat Omori who was crushing Sunny while being his imagination? Why do you people speak of determination as if it's the most powerful thing ever? And speak of it as if what OMORI has is nothing compared to it... At best it can only help Frisk make it a tie.

Other than that, people will say Frisk will act the thing out and win...

Even in Undertale, it is emphasized that acts cannot work everytime. You've seen that with Undyne, you can act but they don't work on her at all, so the only option left is to run. Omori doesn't even let you run himself, let alone spare him.

So who's winning?

No one... Save points are canonly a thing in Undertale. It's just gonna be an unending fight.