This will require patience and accuracy.
First change the default settings to 5 misses, to make it easier.
(also make sure its 2 player against the AI)
You will start off as the the blimp for the first few passes
near the top of the screen.
You will automatically hit quite a few targets for these.
Then you become the biplane which is much lower and faster.
MAKE SURE YOU HIT AT LEAST 1 Target or DROP 1 BOMB each pass.
You must learn the angle of the bomb relative to when you drop it.
The speed is constant.
It will start off at a 45 degree angle, and then quickly become much sharper (steeper).
Once you have a good feel for where the bomb will drop compared to your airplane
you will be able to pick off single targets.
Let the computer finish bombing, as it will clear out quite a few targets for you.
If you think you're going to miss, only take one shot per pass.
Here's my sample run clearing it out (with some misses):