I have to say I'm very satisfied with the dragon's performance. It's so jolly good when these hulking beasts generally speaking don't give a shit about the humans' attacks, and even the dragon slayers' attacks only push them back a little. This gives a proper perspective of the past when humans were facing an extinction, no matter what they did, before half of the dragons decided to give them a second chance. Though considering all that, and how the tale ended, according to Zirconis, the ancient dragon slayers were worlds above Natsu & the others. They still have a long, long way to go. In fact since Natsu isn't probably so far below the power level of the strongest wizards at the moment (he has beaten quite tough opponents indeed), it's probable that if he ever reached the power of the historical dragon slayers, he would be unstoppable (as long as he wouldn't face Zeref and Acnologia, at least). The wizard saints, of whom we know nothing about, would be like children before him.