I wasn't able to fight the boss - but I think after some study it should be possible. But I think the hard boss is ok (a boss should be hard, right).
I like the artwork and the sounds. The background music creates a really fitting athmosphere.
But it seems like that you're particles are emulated in local space and not world space. I prefer last one because then existing particles won't move with the player. E.g. if you walk backwards it looks kind of weired. But if you pay attention you ofc can see it also during jumps.
Overall a really good game :-)