No, he couldn't. Normal Sephy definitely won't. WoW has so many people and so much magic and GAR that Sephy would fail rather readily, despite his speed WoW has people who can cast spells with a thought and other people who're blatantly super-human as well. Sephy had to absorb the Life Stream to do anything major. Ragnaros the Firelord, when he emerged from the Elemental Plane, he tore up a whole area of the world and created what is now known as the Burning Steppes as well as Blackrock Mountain. He literally sundered the land and created a chain of mountains and kills just by appearing. And Ragnaros was just one of several Elemental Lords that serve the Old Gods.
Trance Kuja could cause some severe harm if they just let him do as he pleases, but he hasn't shown any kind of mental resistance and there are tons of figures in WoW who could mindrape the living hell out of him or even stop him in time, or speed/reverse age him to infancy/dust. Nozdormu is the Aspect of Time, he exists infinitely across Time as a part of it. Nozdormu knows the exact moment he will die, because the Titans are the ones who showed him, and it was to teach him a lessen that even he cannot change his destined fate. The Titans and the Pantheon are cosmic beings who go across the WoW-verse creating and spreading life across the cosmos and have been for untold ages. Entire planetary-systems are born and given life in an instant by them. And the only ones to pose a threat to the Titans are the Old Gods.
Take out some? They're gonna need everything they've got. Sargeras, the Fallen Titan turned Dark God of the Twisting Nether who commands the Burning Legion destroys the world by just setting foot on it. A number of WoW mages and magical figures can rip people apart with hand gestures. Literally. As in your whole body gets ripped in half. More skilled people like Khadgar can absorb pure magical attacks (meaning non-specific element). I know some FF-verses have Reflect which can reflect spells, but only a certain number. Warriors can do the same thing with their shields.
Ultimecia with her TIme Compression could be a problem, but then there's Nozdormu who literally kept the universe from collapsing by holding together All Time by the skin of his teeth, which had been unraveling due to a plot by the Old Gods in their bid for freedom.
Sin is worthless. One of the least-powerful entities in the FF series. What's the best he did? Ruins the world once every few centuries, gets beaten by a summoner and revives self via possessing an Aeon and its summoner, and got permanently put down when all the Aeon's were destroyed/sealed. Deathwing sunders the whole planet in Cataclysm.