User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-168.169.25.92-20151106203019

Their essence is black goo - as seen in the episode where they traveled through the sewer system and the episode where they dropped a car on one of them.

If that black goo can be contained, and the Leviathan aren't around and about, they are dead for all intents and purposes.

The Archangels could easily separate them from their vessels by killing them in a way much harsher than dropping a car(simply exploding them), and then trap them in that form. Remember the episode where Raphael trapped Dean in a random room that he couldn't leave? Remember when Gabriel reality warped Sam into an infinite loop where Dean died? The Archangels have a variety of ways of trapping the Leviathan in a nonfunctional form.

Castiel could turn himself invisible even before he was God-Castiel, so obviously the Archangels can turn themselves invisible as well. How are the Leviathan going to see and do anything to the Archangels if they choose to slaughter the mass of them while invisible?