User blog comment:Enriant/Heaven and Hell/@comment-26982641-20160602080446/@comment-1975828-20160602094616

Castiel started the war after having obtained a bunch of souls from Crowley. I forgot exactly how many, but he did not challenge Raphael until he had those souls first. Which means, even as a Seraph, Raphael could easily beat him.

Seraphs are said to maintain their power regardless of their connection to Heaven. In season 5, Castiel was a mere angel. Which to me, means that a Seraph can hold its own against a Knight of Hell, as shown in your examples.