Firstly, I am shocked at how you are all having trouble with the game. Now, I know there could be some causes.
I am aware that there is or was a file which included many of the Jam games, correct? It wouldn't necessarily have any help files with it, which could be an explanation. Another explanation, some of you may have mistaken "Living" the hardest difficulty for "N00b" the easiest. You may not have known that you actually shoot the helicopter with Z. You could have been unaware that you can indeed have more than one bullet on the screen at the same time, or that there is no delay between shots and your ability to shoot again. Another, none of you read or at the least understood the "hint" in the topic posted by speedchuck. I will now quote.
Page Up/Page Down-Resize Window
Hint: While using a gun and moving in one direction, tap the other direction to aim the gun that way.
Now you have read all of this and hopefully you understand. If, in the unlikely case you knew all of this, and still found the helicopter boss in the easiest difficulty very difficult, I have no help for you. Pity, perhaps. It's possible to beat that boss within ten seconds, and that's if your ability is less than adequate. You can win without the helicopter even shooting a second missile at you.
N00b is by no stretch of the imagination "difficult." Perhaps you should, after learning the controls and the hint, play again? I assure you it's worth a look, and not quite as hard as you thought it was.