Defeating the Boss on level 3
There is a way to defeat the Boss on the level 3 (with all the bombers) without getting hurt. If possible, clear out some of the numerous bombers. Get to the highest elevation possible. When your fighter is above the Boss on your radar, dive down and fire missiles and the gun. Done correctly, the Boss will not shoot back until your fighter gets too close. Repeat this procedure until he is defeated.
Alternatively, after clearing out some of the numerous bombers climb t to the highest elevation possible. When you fighter is above the Boss on your radar, dive down and wait until your fighter is locked on the Boss. Fire the missiles and go through the Boss. Your fighter should explode. When the game restarts the Boss will still have the damage inflicted by your attack. Repeat the steps until the Boss is defeated.