This was fun! I like Undertale's combat, so of course I was gonna find this neat.
I will say though, I feel like the difficulty could've been spread around a bit. I beat both the first and second bosses on my first try, while the third boss took me like, 10 tries.
The third bosses spiral attack was what kept getting me. Moves like that are why Undertale has invincibility frames.
The parry is an interesting idea, but the timing seems really finicky, and way riskier than just moving out of the way. You could've made it relevant/necessary by making attack patterns that require it, but I'd recommend making the timing more forgiving, along with adding some indication that the parry cooldown is up.
Also, one other thing, (a common mistake that even I have made in the past) I think this game very much expects to be ran at 1920x1080, which means that everything gets scaled down a bunch and moved into the top left corner.
Your could fix this really easy, you just have to go into the project's settings, then go to "Display", "Window", "Stretch", and then change Mode to either "viewport" or "canvas_items".
I hope that helps!~ P.S. I attached a screenshot for convenience!~