I really liked the demo, the sounds are very good, and the gameplay is super pleasing.
A few suggestions:
1) Try to use a singleton to play the music, so it doesn't restart after you change the scene/level.
2) Improve the hitbox of the big dark fly enemy, is you shoot him in the legs, he won't get hit.
3) When hit by an enemy, try to put the player "invincible" for a few seconds, as you can die very quickly if a monster is close to you.
But good job! :) I want to play the full game!