The sprite art on the characters and background is well done. The player's running animation looks quite smooth. Did you make the art assets yourself?
It's a bit too easy to kill the boss with your attack since you just stand there and spam the attack next to him. It would be cool if there was a different way to attack him that would be more challenging. Like maybe you can swing your sword at the bullets to knock the bullets into the boss, which would require proper timing. That's just one idea.
Nice job on making a bullet hell for the first time!