User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-190.92.7.204-20150612220839/@comment-5961531-20150614011611

Technically Alastair didn't say "if she could", he simply said that she wouldn't stop at seven Angels, she'd kill a hundred or a thousand. Logic would tell me that she could kill an Angel, or at least knew how to. During season four it's said more than once how the Angels are losing the fight against Lilith. She broke at least half of those seals herself if I recall, which means she must have fought and defeated some Angels. Again though, she may not have had the means to actually kill any of them, just exorcise them and send them back to Heaven. Anyway, my opinion is that Lilith was more powerful than Cain and as I've already stated, Cain having the means to kill an Angel doesn't make him more powerful.

Castiel was a Seraph from season six. Plus him being a Seraph was practically irrelevant when he fought Cain as he had yet another borrowed grace, most likely from a regular Angel. In which case, Cain defeating Castiel isn't really that much of a big deal when a weaker Demon (Alastair) already has. Twice.

"Deadliest" doesn't equate to most powerful.

I doubt the knife would work on Lilith either. It didn't kill or seem to phase Alastair and Lilith was far above him. Cain may just have an exceptional pain tolerance.