Achille's flaw would be his confidence. In the story, he is practically invulnerable and cannot get hurt by physical means. He can be killed immediately, however, if hit in the back of the heel. This makes him cocky and arrogant when it comes to battles. When it's time for his final fight, Achille's confidence in his survival overweighs the possibility that someone will find out his weakness. When in battle, he gets shot in the heel due to his confidence blinding him from the fact that he isn't completely invincible; he can still die.