It is the element the eight-headed beast is weak to and which can seal its wounds and prevent the growth of new heads. That is the firepower 'needed'.

Any other damage done, is of course, welcome for weakening it. But it cannot be relied on to kill it in the same way.

[He gave a small sigh.]

G'raha, this venerable one is aware you are talented. You do not have to attempt to prove it.