He only got two shots off. A practice shot (which almost hit), and the second shot which brought down the dragon. Excellent marksmanship for Bronn.
Still, since they had the scorpion wagon, where was the dedicated AA crew for that weapon? Why weren't they firing it?
Also, since they had the AA wagon, that meant that they anticipated seeing a dragon. Which means you should have anticipated the napalm attack on your bunched-up troops.
The fact is, without similar air support or very effective anti-aircraft (one scorpion is not sufficient AA), the Lannister army was toast (pun intended) from the get-go.