1. attack their inceptors with fighters with drone escorts to take some of casualties instead of just the fighters taking them all saves you money as drones are much cheaper,mixture of 2/3rds fighters 1/3rd drones work ok 2. once the interceptors are destroyed attack the helicopters they will be no match for your fighters if no interceptors are remaining to assist in defending them, once all thier air wings are destroyed you wont have near as many casualties attacking the forts and cities, somtimes mid range missle divisions can knock out targets with no casulties at all on your side depending upon the weapons the garison has or either use land forces and or helicopters and or drones to take out cities and forts etc. ( prooven way works for sure)
another way to get rid of c3 air strenth that ive never tried because i have tons of fighters but should work in theory is form land divisions with just a few tanks and tons of offensive anti air