Leon was unsure what exactly had happened to him. He had just felt high morale and that his strength had continually increased. With these two factors, he had waved his magic sword repeatedly, each stroke getting stronger, on the dragon's head. This was how Leon managed to keep the creature, which was a few times bigger than him on the ground, where the dragon was unable to fly up.
To prevent the sword from losing strength, he habitually crossed the sword and used the sword ridge as a fly swatter, as if he was really about to swat a giant fly. Compared to the analogy, though, it was just that the "giant fly" here in this analogy was not actually a fly but rather a giant, evil dragon.
