Very nice game! The concept is simple but works. My opinion:
The graphics are simple, the background, targets and clouds are good. I would recommend improving on the boss and player's graphics. Also, add some animation to the fire. Finally, work a bit the graphics for when the player's aims his weapon to the left, it looks almost as me with a dislocated arm.<img src="smileys/smiley36.gif" border="0" align="middle" />
I felt the dynamics were good, and maybe you could, as vtrix said, lengthen the game/level a little more. As for the 2nd level, where you try to destroy them before touching the ground, I would recommend instead of the boxes appearing from the sides of the screen, maybe make them fall from the sky, with the proper speed so the player can move and destroy them.
Overall, I think you are doing a good job! <img src="smileys/smiley20.gif" border="0" align="middle" />