He also knew Dagon's Protonymic and neonymic, had a weapon forged from Dagon's essence, and possessed the saviour's hide. While he did win, it was because of the artifacts and knowledge of the protonymic and neonymic.
Even so, defeating him was a far greater acheivement than CoC getting Martin to call on Akatosh to defeat him.
To be caught by surprise by one's enemy is a great failure. When scholars already know how to open oblivion gates (before the oblivion crisis), they have no excuse for being unable to close them.
Telvanni are the ones with arcane knowledge, if they weren't such politically driven [censored]s they could have stopped it, I blame the Telvanni