For practical purpose I'm just going to comment as if this were a finished game.
1) This is categorized as a horizontal flight shooter, but it actually plays like a vertical flight shooter.
2) Very distasteful mix of graphics styles. The characters are poorly scaled-down artwork, the trees look like they were ripped out of a SNES game, and the background is just some random oversized texture. Some visual effects are poorly complimented with ugly 1-pixel lines.
3) It is not clear enough that the player needs to use the arrow keys to move the player to start the game. Perhaps a popup image of the directional keys would help.
4) Dodging can be interesting but the character moves annoyingly slow and the hitboxes are weird so that I get hit even if I don't make visible contact.
5) Shooting is boring because it works best to ignore the basic attack and spam the charged attack. It's hard to run out of mana.
6) When I reached the first turn in the path, instead of giving me an option, the game continued moving forward off the path and into the blue (ocean?) and all the enemies went offscreen as they continued to shoot at me. I eventually managed to kill the enemies I couldn't see and eventually returned to a path and confronted the boss who I manged to beat without even collecting a single red, green, or blue crystal.
7) It is not clear at all that the green crystals level up the player. Perhaps there should be some sort of level indicator?
8) It took me a while to figure out that spacebar was attack. (I assumed attack would be 'A' since it was highlighted.)
9) It took me some time to figure out that I could charge attack by holding spacebar.
10) Everything else was easy, but that final boss after the boss was practically unbeatable. It is unclear whether he is supposed to be beatable or not.
11) I feel like this game has an unnecessarily large vertical resolution for a browser game. This may limit the number of people who can play the game.
12) I think the music sounds more compressed than the sound effects, making them not fit together well.
13) The game shifts from easy to hard and obnoxious just by accidentally letting a few enemies gather, since their spears cannot be shot through and they fire a lot of them. I find that to be an annoying gameplay element.
14) Whenever I 'die', even without reaching the bosses, I get sent to the final boss ending scene. Why?
This feels more like an alpha test than a demo.