5/5, great game for undertale/ enter the gunegon fans. Keeps a high level of difficulty while being engaging with the reflect mechanic.
The obvious route with this would be to make it into a dungeon crawler (like gungeon) OR to add more bosses in and tell a story (like undertale). You guys got a lot of freedom if you choose to keep going with this game!
I would also consider adding in a few mechanics (and adjusting the skill curve in porportion), such as (destructable) cover, and maybe a mini "blank" effect similar to blastfamy from ETG. Also, when going to the right side of the boss, the sprite "flips", but the sword attack is now underhanded, which looks and feels a bit jarring.
Lastly, the sword takes just a little too long to charge in my opinion. If you want to keep the difficulty curve, you can make the boss harder to compensate. From a player perspective, its around 1.5 seconds, but it really feels like 2.
With a slightly faster charge time, the player can be more aggressive with more risk reward (dodging back to reflect 1st wave, than having an option to go into and reflect the next wave).
Other than that, consider a tutorial / easy boss to help new players ease into the game. I feel this would really help you guys, as right now it is really easy to put the game down early, and I fear only undertale/gungeon fans (and similar frustrating yet rewarding games like Super Meat Boy) will stick around.
Good luck and keep up the good work! - Yaniv