This is what i'm saying here. Notice how in the first pic they are still trying to determine if it's the dragon because it looked like a bird.
By the time shot they realize it's the dragon and they shoot their first shot and Danaerys dodges it. We've seen her dodge smaller arrows before when she took out the Lannisters and the Tully army.
AFTER the first shot is missed that's when you see the guy below says "TURN IT" while they try to reload the projectile.
As Euron is once again struggling to find where she's hiding she comes down again from the cloud and takes out the first couple of ships
Again you see and hear them saying "turn it around"
Then as she headed to the walls she flew low as if to bait the Lannisters on the wall to shoot bad shots low and all she had to do is come up and dodge them all in one move. Once you miss it takes time to reload and then point the damn thing towards your MOVING target while you are stationary.
Again, doggy i know it's fantasy and Danaerys was meant to win but it aint too farfetched. The farfetched thing is that the dragon had that much fire to tear down an entire city os brick homes