He asks this question to emphasize that the army of Hezekiah does not have the resources to fight. He also uses exaggeration to mock Hezekiah's army. AT: Possible meanings are 1) "You could not defeat even one of the least of the king's soldiers." or 2) "You could not defeat a group of the king's soldiers commanded by his least important officer." (See: [[rc://en/ta/man/translate/figs-rquestion]] and [[rc://en/ta/man/translate/figs-hyperbole]])
He asks this question to emphasize that Yahweh is behind his success to obey the command to destroy Israel. AT: "Yahweh himself told us to come here and destroy this land!" (See: [[rc://en/ta/man/translate/figs-rquestion]])