There are three distinct waves you need to survive through. Remember this can be done on simplistic difficulty.
First wave is dive bombers. You'll start out above them, but remember in order to render the divers defenseless you need to shoot at them from below where their gunners cannot hit you.
Second wave is torpedo bombers, and you will be incorporating the same technique here of getting below them to kill them.
The third wave is not so much a wave as it is a kill limit. You need to kill four (4) fighters. The problem isn't them messing you up as you should be on simplistic difficulty so much as it is you flying into one thus killing yourself.
If your plane has taken a lot of damage hit the button and move to a full health plane. Simple.