User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-203.81.71.69-20140507062832/@comment-25358468-20140508222734

I see, but that isn't absolutely confirmed. It's also said that werewolves can only be killed with silver and the Colt, but Cas has been shown to kill regular monsters using his white light ability, so it's flexible. Also, Leviathans weren't introduced until Season 7, and they were locked in Purgatory so long they were out of the picture, and turned into a game changer when they escaped. I'm just saying, I don't think Leviathans can kill archangels, nor vice versa. I do think it's up for speculation is all.