User blog comment:Twilight Despair 5/Fans for Season 8/@comment-3481504-20120408172318/@comment-4920092-20120410173625

Castiel was only overwhelmed because ALL of them were INSIDE of him at once. He wasn't actually fighting them, vessel vs vessel. And it certainly wasn't a fair fight, because it was him versus every single Leviathan at one time. It's not only possible, but highly probable, that in a one-on-one match, Cas could have taken a Leviathan, even Dick Roman.

And Jesse was far more powerful than any demon, even Azazel. He could speak, and reality obeyed - it's the closest thing to omnipotence shown on-screen in the Supernatural Universe, so it stands to reason that a half-angel would be similar in power.