I really liked this game! The puzzels were fun, and the bossfight was delighfully difficult. I would suggest a couple of things:
1. Make the player hitbox smaller than the sprite. Nothing kills the will to play a game as much as being one hit away from defeating the boss and having a projectile just hit you. Keep in mind though, you might break the puzzles if you make it smaller in every direction, so i would recommend having a "hurtbox" for taking damage, and a hitbox only for collision.
2. Make the boss easier. For a short demo like this, having a grueling bossfight like this at the end is a bit too much. I enjoy hard bosses, but this one took me almost forty minutes, more than the rest of the game. One way to do this is to increase the delay between the boss deciding where to fire and it actually firing.
Overall, really nice demo! Ended up playing it twice to get the story because i missed the language options at the start, and enjoyed every second of it!