Nice concept, haven't seen anything like that. It is hard as hell though, I didn't manage to finish a level :)
It would have been nice to have tutorial level, and introduce the mechanics one at a time:
For instance in the first level you have to shoot a few enemies so you can see their spawning and move pattern.
In the second one you force the player to change weapon.
In the third one you force him to move.
etc...
Here the player has to figure that one all in one go, by dying a lot, and it seems unfair because the rules were all thrown at him the same time.
But the concept and mechanics are great and well executed, so goodjob :)