I love the art, it's a great style!
I found the first 4 bosses quite easy tbh, I didn't have to parry a single time. The final one was more challenging then, but also there I didn't really need to parry, just dash away and use the time in between attacks to hit it.
I'd love for the parry to be a kind of build-up for a more powerful attack, with some cooldown then. This would make it more useful and also provide variety in your own attacks.