User blog comment:EmpyreanSmoke/Weaknesses/@comment-69.64.58.255-20151001035430/@comment-24834479-20151001040345

Here is our argument. Castiel and Death claimed that the Levithans could "chomp the ENTIRE aquarium". They also both said that he Leviathans predate, and can kill angels. Archangels are part of this "aquarium", and are themselves a type of angel. It has been confirmed by Death, Castiel, and others that Levithan can kill angels. Archangels are angels, so unless it is stated otherwise on the show, Leviahans can kill archangels (being angels, which Leviathan can kill).

Also, Dick Roman said that he would wipe out the entire species of demons. This confirms that leviathans can kill every single demon. Lilith, Azazel, and Alastair are all demons, which have been confirmed vulnerable to the leviathans.