They should gather all available (high level) manpower from the cave heavens and paradises. All those sleeping ancestors. There's no other danger around than this single ink clan member, so the humans have the luxury of concentrating all of their fire power in a single point. If there were suddenly 20 ink clan members in different territories, it would be a different thing, but there's only this one, at least that they know of and that's about to get free. It seems foolish to use limited power fighting to the death as opposed to using all power in a more safe manner.