This is veeeery well designed like everything just works together really well. The animations, the way that it's a boss battle but it's slowly stacking on more mechanics , and the audio chosen too. This just has a lot of polish for a game created in one week and I just wanna say good job on that!!
The only feedback I have is that I wish there was some sort of sound effect to signify that the player's heavy strike is being charge because a couple of times I was worried my input just wasn't being read but I didn't hold the space bar long enough. Not only that but the heavy strike doesn't feel heavy if that makes sense? All the normal attacks have a general woosh sound but a heavy attack should feel heavier than a gentle "woosh" in my opinion.
Also, odd little bug I found was that for some reason the background music keeps stopping at certain points leaving an empty silence before starting back up. I first I thought it was meant to be in there when your health was low, but as I kept playing the game I realized that wasn't why that happened.
I'm still trying to beat the game as I'm making this comment and I finally got to the last phase on full health and died because I was so caught off guard, but in a good way. I feel like bosses always have phases that catch players off guard causing them to have to start over again and I think you pulled this off extremely well. I usually don't like playing action games where you have to die a lot in order to "get good" but I feel like the attack patterns were pretty fair and by just getting the timing right I'll eventually beat the boss.
TL;DR 10/10 I'm literally in awe of how you made something so polished in a week. Great job!!