This idea is amazing, and the execution is pretty amazing. (Good juice on shooting) Definitely one of the better games so far. The main mechanic is so awesome that I'd be disappointed if this doesn't get mentioned.
However, the difficulty is a bit too high because the controls aren't very refined. Mostly, the difficulty comes from the player falling quickly, and not having a high enough jump to move between some of the platforms. Small platforms with too much vertical room in between increases the movement problems. I would have played for a long time, but those simple problems would need fixed first. The more technically complex problem to solve is when you shoot a platform your character is clipping through, and the platform becomes solid, but you still fall through it. I died to that more than the enemies.
As for the design, I think the enemies should prefer to enter on the top half of the map, for a few reasons. First, enemies that come from the bottom are very difficult to kill, since you cannot shoot below the platform you are standing on. Having lots of easy to kill enemies gives the player more fun things to shoot at. Next, having enemies above the player increases the likelihood that a player will shoot a platform above them, which is a fun way to die. (Dying to a personal mistake offers players a chance to learn, and it makes people want to try again, which is always good.)