User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-184.170.2.32-20120516070654/@comment-98.113.182.208-20120516155800

Here is the exact passage from Kevin's translation:

"Waste not thy time nor your breath upon the leviathan herd. Point thy blade at the heart of their master, for from him springs all their messages. Leviathan cannot be slain but by a bone of a righteous mortal, as light and good as the Leviathan are hungry and dark, washed in the three bloods of the fallen: a Fallen Angel, the ruler of Fallen Humanity and a father of Fallen Beasts"

They cannot be slain BUT BY etc. etc. which means they can be slain BY only the right components and ingredients.

So that solves the issue here.