How many Anti-Air Battalions do you need in a Divisions to completely neutralize the enemy's close air support? Is it even possible?
Cannot prevent CAS damage, just reduce it. But AA can reduce Air Superiority -50% defense stats. It will need 4 Medium SPAA with +5 on AA (total AA up to more than 200). And this apply to division of any size. That's why 40w is more easy to defense because you don't have room or cost for 4AA in every 20w division.