This is a nice start, I had fun playing through. (time=184.133, after a couple tries).
Two things I’d change: The jump retriggers if you’re still holding X when you hit the ground. That’s a common mistake in platformers, and usually not a big deal, but here we’re paying by the jump so it’s bothersome. And second, some of the monsters appear on top of each other so you don’t know how many there actually are in one spot. That caught me a couple times, and like, there’s no good way to defend against it aside from remembering which monsters those are.
Overall it’s pretty solid! I could see this fleshed out as lots of small levels, where you’d really have to manage your jumps and shots to avoid running out.