This is an absolutely wonderful submission. The art style is great, the effects from the sword-slashing feel cool. And winning the fight made me feel super satisfied. Honestly, if this became a full game, I would play it. That said, I have some comments for improvement.
The first one is small. I just like dark souls a lot, and I thought the controller controls felt weird because they didn't follow the formula. I know it's not a 1 to 1 mapping since they have different controls, but maybe an option to go to a more Soul's style control scheme.
Secondly, just some juicy sound design would be nice. Heavy hits should sound and feel heavy for both sides. Combos should have some kind of stacking sound effect that gets better and better the longer you pull off a combo. Just stuff like that.
Finally, I just think this comes with time and testing so it's not really a rag on how the game is, but rather balance tweaks. One balance thing I would look at is telegraphing and the end lag of the boss's move set. I think a move's power needs to be directly proportional to how much build-up the move has and how much end lag it has. Some of his moves do this really well, and some don't. Another thing to look at in balance is a staggering system based on the weight of moves. It didn't seem like this existed, but rather if you got it, or the boss dad it was a stagger. I think the boss and the player should have a certain amount of stagger resistance so that smaller moves don't just knock you over. Maybe the stagger doesn't clear instantly either so you can combo a lot of small moves together to get a stagger. I think with this you could find a way to balance it so that the boss doesn't have to become invisible to escape the player just combing him into infinity. That was probably my least favorite part. Because sometimes he would become invisible and I wouldn't realize, my sword would go right through him, and then he would slash back and kill my combo meter.
Sorry for so much feedback. I think when an entry is this great, it's very easy to talk about. Great job and I hope you guys do well!