didn't know that weakness. They only knew that their one weakness was written down somewhere.
Witchcraft and decapitation slow them down at least. Prior to learning their true weakness, Sam and Dean tended to defeat them with a combination of borax and decapitation.
Big Eater: As in, eating humans and everything else.
As in, 7 billion humans is not enough food for them.
Broke Your Arm Punching Out Cthulhu: Turns out killing Dick Roman, the head Leviathan, drags the nearby people who killed him into Purgatory.
Corrupt Corporate Executive: Dick Roman, the man whose form was assumed by the head Leviathan, was one of the richest men in America. Since the death of the real !
Now remember that, due to their Healing Factor, the sheer act of doing so becomes a Fate Worse Than Death since, if they slack off at all whilst devouring their own flesh, it will simply grow back and they'll be forced to do it all over again!
Dangerously Genre Savvy: Despite being Smug Snakes, Dick Roman and the Leviathans have their moments. For example, when the Winchesters are gathering what they need —Dick summons the current King of Hell, Crowley, and traps him in his office, therefore ensuring that the Winchesters need to go into his office before they can finish the weapon.
Gets better in the finale, as Dick has several other Leviathans copy his identity by copying DNA from the since-deceased real Dick Roman's arm, so the Winchesters don't know which one to attack.
Demonic Possession: They take control of Castiel, or at least hijack his vessel Jimmy, at the end of the Season 7 premiere. Noticeable was the change in tone of voice from to one that sounds more like an insane version of normal voice.
In the following episode, Jimmy's body is about to be destroyed by their power, so they disperse, mass-possessing an unknown number of people across the country.
This weakness is fairly hard to use against them because of meeting the above conditions and that they managed to keep it a secret until "Reading is Fundamental." As for their other weakness, see Weaksauce Weakness below. The only other known way to kill one is have it eaten by another Leviathan, or being forced to eat itself.
Until "Reading is Fundamental, " even