*cough* Odin/Gilgamesh *cough*

Storywise, I can't say cause Sephy has been DBZ'd in the Compilation cause he was not nearly that powerful in his actual game so this would come down top just how strong Junctioning makes you. It's a toss up really. Also I would point out that Squall also has quite a bit of combat experience seeing how he's a child soldier not to mention he lacks Sephy's main Achilles heel which is his ego at being the strongest. Chances are, Sephy would toy with Squall long enough for him to pull of his Limit Break and I'm pretty damn confident that fully Junctioned, Renzoken would be enough to take him out.

Gameplay, it's hard to say cause if we're going Squall vs. character Sephy, it really depends on his Materia set-up and even that is going to have issues. If we base it on what Sephy used in the Nibelheim incident then Sephy is screwed. If it's his final boss form, he is still totally smurfed cause Squall is going to smurf him in every hole nature gave the silver haired fiend.

The only thing that will protect Sephy would be an author intervention, and considering how much they've gone to great lengths to re-imagine the character as the most powerful being in the FF mythos it sometimes seem, I'm pretty sure Kitase, Nomura, and Nojima would jump to take the proverbial bullet for Sephy and canonize him as being unstoppable except against Cloud.