Anti-Air, often abbreviated to AA, refers to assets that are able to effectively combat aerospace fighters or VTOLs. Good AA assets often bring a combination of high pitch-up limit for shooting at high altitude targets along with a loadout of weapons that can deliver damage effectively to aircraft.
One of the biggest challenges to driving air assets off the field is being able to hit them. Regardless of whether you are using a rapid-fire weapon such as UAC2s or a powerful, single-shot weapon like a Gauss Rifle, the trick is a matter of finding the correct distance by which to lead the target and also knowing how to compensate for the convergence as well as location of your guns. There is no easy way to learn this other than by practice and experience.
A good choice for performing early anti-air work is the Partisan A, as the LBX5s are very dangerous against aircraft without sacrificing a lot of damage potential against ground targets.
Another way to force bombers or VTOLs off your team is to bring an aerospace fighter yourself. Choose one of the Fighter variants equipped with strong anti-air weaponry, such as the Air UAC2, Air Thunderbolt or Air Lasers. Be careful when pursuing your targets though, as they will often do one of two things: either they will fly back to base to the safety of the base defenses, or they will fly over their team, who will attempt to destroy you.