I actually had a lot of fun with this and played it for a while despite the difficulty. I feel the combat was enjoyable and has potential, but at the same time could be frustrating and needs a bit of work to feel fair. I would keep the lethality but perhaps more/clearer telegraphing of enemy moves and/or more leeway in timing the dash to dodge attacks. Maybe also sound effects to signal the enemy is about to attack.
Your game actually benefited a lot from not having a HUD in terms of immersion, and I appreciated the contextual hints to teach you how to play - they worked really well. But I did miss indicators of the enemy health (especially the boss), and I wonder how you might better show that whilst still keeping this HUDless - damaged/bleeding sprites, or some other visual indicator?
I really enjoy this kind of twitchy gameplay and find it really addictive, so I wouldn't lose that, just make some tweaks. There was also a little repetitiveness in terms of the enemies you face, and I found myself skipping some of them by shifting between worlds or just dashing/jumping past them. The visuals are really really nice and I'm jealous of your living/dead animation as well as the "shadow" behind the player character. How did you do them?