Of course he would have ignored that part as it was too stupid for anyone to take seriously. From previous scenes in the film it was clear that the dragon's flame had a longer range than a mere hunting bow, plus with a weighty explosive charge on it when would have at least halved an already short range. He could have flamed them before the human even nocked the arrow. It also made no sense to kill the male dragon even if they could, because now with nearly all of the humans in the world already eaten or incinerated, he was now eating the smaller females. All the human survivors had to do was keep hidden, and once the big male ran out of female dragons to eat, he would have hibernated for another thousand years and what was left of the human race might have recovered. It is a shame that with such good dragon CGI and an interesting storyline, this turned out to be such a dumb movie (and most film critics agreed).