I loved the sprite work, the run animation in particular is excellent! I think there were a few issues that stopped me from really enjoying the game though. Movement feels a little too fast and slippery - it's better than slow and frustrating, but I think the speed needs to be reduced quite a lot. Jump being on Z really hurt my hands, same with having to spam click x - the layout of the controls really gave me some trouble. Jump being on the space bar would help I think, but the biggest win would be making it so you can hold the button to shoot.
You also need some feedback - I need to know when I get hit (look at hollow knight - the whole screen freezes, there's a big particle effect, and sound effects, plus the sprite flashes - I know you can do that last one at least, because you do it for the boss!)
The boss itself was generally fine, but it felt really unfair that it can shoot through walls but I can't. Coupled with the lack of feedback when the player gets hit, I sometimes felt like I just died out of nowhere!
There are a few bugs I'm sure you're aware of. I can just sit in the lava indefinitely, I can double jump sometimes, that sort of thing, but I'm sure you'd clean those up if you had more time :)
The music fits well, and the sound effects are nice :)