A turn or two ahead, during good weather (then you get the largest area of recon as opposed to one square with bad weather), use an experienced fighter to move to the objective/city/airfield to know what you are dealing with. The recon air unit may take a strength point or two loss from AA--no big deal if it was strong and experienced. Then, get your most experienced paratrooper unit airborne and ready to be dropped--this takes planning and TIMING. Move your combat forces close to the complex and hopefully this will draw out defenders to attack you and weaken the defense of the objective. Experienced armor and artillery are the best. Then bring in two bombers, one to hit and suppress the AA, the other to hit the artillery unit. Move and position your paratrooper on the square next to the AA or Arty--get ready to drop. The enemy's turn will be the AA firing--it may try to shoot at the paratrooper--which leaves the bombers to hit the AA and Arty again! Any unit of yours next to the enemy unit will inhibit it from fully reinforcing--in fact inducing it to do just that is fatal as it is possible none of your paratrooper units or air units will take fire while the enemy desperately tries to reinforce to survive and delay! If this happens you should be able to, on the next turn, re-attack with your bomber, move it away, bring in a second bomber and hit the AA yet again! Meanwhile, you attack by ground with strong armor and arty--a pincer movement from two sides is best if possible. Infantry--Pioneer are best--can hit AA well though an experienced armor unit, perhaps, works a little better. Hit the Arty from the ground. Now your paratrooper has dropped and inhibits AA or Arty or both from fully reinforcing and can attack adjacent units. Surprisingly, the AA, unless experienced and large caliber does relatively little damage to your soon-to-drop paratroopers or your air units--use fuller strength, experienced units for such an attack.
It is doubtful you can use paratroopers alone, deep behind enemy lines--which is always risky--and survive, though you can occasionally find an isolated airfield. I used to employ two paratrooper units but the area gets crowded and usually one paratrooper unit does the trick while allowing the other infantry to do their thing--pioneering or bridging--on the ground.
Combined arms and timing are most important.