you need a very fast thumb, you need to keep pumping out bullets at a very fast rate, the first level is pretty easy, make sure you don't hang to far to the back as enemies can come from behind, make good use of your R-ball, keep it on your front for the first 2 levels, dont fire it off it's better stuck on your ship, it's also indestructible, if you run into the smaller enemies head on it will kill them, for the first level boss fire the r-ball into the middle where the little face appears, then move the ship into the top corner, the r-ball will lodge in the middle, just keep shooting and it will kill it soon.
as for the boss on the second level you need to position the ship just over the eye that appears, the snake will pass right over you bullets will 95% of the time miss you.