Well...
As so many before me have said before me have said, far too hard! Seriously, this is one of those few survival shooting flash games in which the game actually gets easier the longer you last, instead of the other way (you have to survive all the whole pistol and headshot hell before you can actually have your shotgun with so few bullets that don't even last one level).
Also, thanks for at least adding the chance to have headshots. Unfortunately for all of us players, all of the enemies can either run like Hell, or they constantly dodge them.
The two best things of this game are the awesome intel officer in the green suit and simply the fact that this is probably one the most realistic sci-fi survival shooters: for actually how long do you think one man with a pistol, shotgun and a small array of weapons and bullets can stand against an army of aliens? If your objective was trying to make one of the hardest flash shooters with the highest and one of the most intelligent enemy AI's starting from the first level, you have succeeded perfectly...