First of all, I want to say I really loved the style of this. Though the game was "short", it really laid the foundations for a beautiful full release. I will say, I didn't get the mechanic of combining bullets until I had read the GDD, but I think conceptually it could be really cool with a bit of work.
With this being a GJ focused on a PROTOTYPE, I think you did a fantastic job. The only criticisms I have are that it was impossible to understand what was going on solely by playing the game (but as mentioned, remedied by reviewing the GDD) and the enemy spawns could use a bit more work to ensure they don't spawn directly on top of the player.
I thought the combo display was awesome, and gave me some of the same feelings as DMC and God Hand! With a bit more sound design to give it some real oomph! For example, if there was auditory feedback on some of the UI elements, I would have felt like a kid in a candy shop xD
Overall, I think that this was a very interesting and unique concept that was executed very well. I didn't encounter any game breaking bugs after roughly 30 minutes of playing, and only have a few small criticisms. I would love to see this as a complete game in the future! Keep it up :)