Basically, in worlds like this, Heroes either fought the Demon King alone or formed squads to do so. The reason was simple.
Humanity lacked both troops and firepower.
Generally speaking, the Demon Race had ample manpower and powerful firepower, or could command magical beasts to fight for them. By comparison, the forces humanity could mobilize were much smaller. Especially in an era like this, with neither large-scale weapons of mass destruction nor easy-to-use weapons, on battlefields where sheer numbers were used to grind down the enemy, the side with more soldiers and less fear of sacrificing them naturally held the advantage.
Thus, the best way to offset that advantage was to concentrate a small force of Elites and launch a decapitation strike against the enemy.
Regardless of whether the enemy was the Demon Race, Tyranids, Greenskins, or some other monstrosity, this tactic was useful.
The prerequisite was that the Hero had to be powerful enough.
So, to put it simply, humanity did not avoid a war of numbers because it did not want to play that way. It simply could not afford to.
If they could have, they would have done so long ago.
Then, would killing the Demon King solve the problem?
In fact... no.
Killing the Demon King would only leave the Demon King Army without a leader. They might start fighting among themselves over the right to rule, buying humanity a little time. Therefore, the best solution was still to annihilate the Demon King Army completely, killing every last one of them. Humanity simply could not do that, so they had to settle for assassinating the Demon King.
After all, training a warrior from birth until they were capable of entering the battlefield took at least a decade. The intervals were too long. Human civilization in this