The gameplay gets boring pretty fast...
Ignoring the unfair difficulty made by the enemies getting upgrades waaaaaay faster than you can, and their shield restoring much faster than your own; the whole thing is just kinda dull. You sit there without even moving the cursor and holding the button, wooh...
I like the premise, but overall it could use some work. If you do decide to make a sequel, Give some thought to Megaman, each boss has a special power, a version of which you acquire after defeating them. Seems to me like it would be perfect for this.