User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-5619706-20121108182206/@comment-3481504-20121110214001

Actually, I'm going to have to disagree with you on that one, General. Castiel said "Leviathan can kill Angels. There's a reason why Father locked them in Purgatory." So clearly, any Leviathan is capable of killing Angels.