This is really cool, I love the parallax effect of the shadow and it's a great concept to fight monsters in the shadows!
I think making the monsters' shadows slightly darker and clearer on the background would make it easier to avoid running into them, but I love the idea! Maybe the player could get a power where they can see outlines of the monsters, a bit further into the game?
I died quite a lot so didn't manage to get very far, but I love the art and design! Feels really fun and impactful to play.
P.S. we're looking for another 2D pixel artist for expanding our game if you're interested in joining, I love your work and I think it'd fit pretty well (though ours is top-down rather than side-scroller) - let me know if you might be interested!