Easy Win
Can't get past that first boss? Well, once he appears quickly go to the top right hand corner and just sit there. He'll eventually get bored of shooting at nothing and fly off the screen.... You beat him!!! Now wasn't that easier then expected?

Defeat Boss #4
To defeat the fourth level boss, first make sure you have atleast 3 spare ships. Then, with these ships, ram the boss 3 times. With the fourth ship, hold rapid fire and shoot him for about 10 seconds. He should blow up and give you the bonus level.