[Imp]

1) In most cases a dug in AAA unit especially if its in good protective terrain like rough has much more chance of surviving than a fortified one. As said attacking with standard infantry weapons is slow so just get any sort of AT weapon in range. Generally not worth attacking if you moved wait till next turn, more accurate more chance of a kill or damage as you hit a vulnerable spot like the vision slit or door.

2) Things that effect in particular accuracy & hence range for AAA fire by guns machineguns etc.
One of the few units where I will check that I have not bought a lemon in WWll

Experience - not a lot you can do as normal the higher the better.
Unit characteristics - weapon accuracy & range, firecontrol & rangefinder.
The higher these are the better as your hit chance will improve & hence the initial range the gun wiil engage increases.
Target - targets size makes it easier or harder to hit & its speed effects how often it is shot at.
A large slow target is easier to hit & if it stays in range will be fired at more frequently in its move with the increased chance to hit it as its acquired. Put another way unsurprisingly small fast planes are harder to hit than big slow ones.