I love the originality of this game, and the concept of choosing what powers to give your opponent is quite unique and interesting. I do personally believe that the main opponent is a little bit to unforgiving in regards to it's challenge level/difficulty scaling, but it's easy to see that this game takes inspiration from other hard core bullet-hell games like The Binding Of Isaac and Enter the Gungeon so I see where you're going with it. I think that I would mostly prefer it if he wasn't so darn tanky. Personally, I don't like it when a lot off a bosses difficulty feels like it comes from how utterly long it takes to kill them.
You put a lot of polish into giving the enemy good "tells" for each of their attacks, and I loved the fun narrative. Both of which really impressed me. Great job!