Aerial combat is carried out in the air and is termed dogfighting. Unlike fighting on the ground, there is no cover in the air - though some maps do feature obstacles that cannot be flown over. The only options are to stay and fight, flee, cloak (Neo only), or loop-de-loop (Angel only). Just like shooting when on land, shooting in the air slows down Mechs as well. Ironically, sometimes the best way to survive a dogfight is to land and flee that way. The cover structures and trees provide on some maps can be used to hide.
Note: Online you may experience some "lag" or "latency" when dogfighting. The link below will explain.
To sum it up, you have to think to yourself:
A) ALL moving units/Mechs you see on the map are actually AHEAD of where you think they are.
B) You are not as quick as you may think and you have to give yourself about a second headstart when getting away, or shoot ahead of other Mechs when they are moving or else you won't hit them.