My first impression was that both the boss and the player might need to have longer i-frames after being hit. It felt a little cheap to be able to spam hit the boss during its first phase and similarly, it felt a bit cheap when the boss spam hits the player by simply touching him.
I also found what I think might be a graphical bug? If you dash through the level transition, the ghost-effect from the player kinda lingers on the wrong part of the screen for a split second.
Other than that, playing this demo was great! The controls were snappy and responsive, feedback on attacks and such felt nice, the boss had good overall mechanics and a fair telegraph on its super attack.
Very well done, my friend!