Powerful long range anti air rockets can clear the skies. Also works against infantry too.
Offensive: This has about the longest range of any AA unit so deploy behind your forces and keep several units between you and your enemy and one or two of these will keep the skies clear for the entire round. They can also out range those pesky Snipers though once they are in range it can only take about two hits.
Defensive: Any tank or heavier Recon vehicle will give these a bad day. They are really only useful against Infantry and Helicopters so attack them with anything that isn't one of those two units.