Sound can be very insidious.
It possesses a natural ability to move even the most physically sturdy of creatures. Its effect can be more mental than it is physical and this makes it capable of slipping past defenses.
That said, the stronger a person becomes in this magical world, the better their brain is at being resistant to sounds. It takes magic of equal or greater strength compared to theirs to threaten them.
Alas, the case of the Bullcroc was not quite so.
Its greatest weakness was Sound. It was so physically powerful (for a Four-Star Beast) that its mind seemed to have suffered from it. A high-pitched sound would grate against its mental synapses and cause it to rage while a low-pitched sound would very quickly begin to shut down those synapses and consequently, its body.
Ordinarily, it would require sound magic at the Fourth Circle to be a threat to a Four-Star Beast, but the Bullcroc's susceptibility brought that down one star. I.e one Magic Circle.
