When in battle you have a combat meter that will fill as enemy units are destroyed. When the combat meter fills to
one full bar you can push up on the D pad to have your
unit(s) attack. They will attack all enemies within the green circle. Every enemy ground and air unit kill will count towards the total needed.
This works on ground and air units.
Best way to do this is point your aircraft towards the units
( ground or air ) that are giving you the most trouble. This will usually be either SAM site(s) or Fighter Aces.