I believe there was more than one star destroyer in the movie, what about the one that chased the falcon into the asteroid field? Besides, they didn't start firing the ion cannon until the transports were leaving the atmosphere (can't remember if that was before the shield generator was destroyed). Also, the whole point to destroying that generator must have been to allow the ISDs to fire on the planet as the assault force wouldn't have been affected by that shield.
Well then lets get this in the game. Uav should also be used in the death star map. So the gun turrents can find the fighters. Then they can destroy it from inside the death star. Also this is another reasion to add the r2 unit onf the xwing and ywing. With all the ties and turrents they wont last a sec.