Right, I played some more, and I really enjoyed it, so much that I decided to record it and upload it to YouTube for no reason whatsoever.
After doing a few more tries, I believe the game is really well-balanced, and the difficulty is just right. Hard enough to crush you, but soft enough to let you learn progressively with every death, and improve every time. Although I must admit, the slash cooldown is frustratingly slow in some situations. Especially in that third phase. But that's probably just me not being used to it yet.