User blog comment:ImperiexSeed/ARCHANGEL or LEVIATHAN!/@comment-213828-20120513212716

In a sneak peak clip for "Survival of the Fittest", Castiel says that Leviathans can definitely kill angels which is one of the reasons why God locked them up in Purgatory. Of course, he could just be talking about standard angels, not archangels.

But really, all this is moot since the Archangels are either dead or locked up in Lucifer's cage.