This game is both delightful and infuriating for me. I loved the little eyes while in the ground, and the ground mechanic itself was neat. Being able to bump slimes from underneath was fun and the whole aesthetic was working for me. Holy heck, though, the amount of precision require for some of the areas coupled with the super-fast character movement had me about ready to smash my keyboard. I finally had to give up because my keyboard simply isn't precise enough to edge up to a red block to squeeze a jump through a one-tile opening.
I think dan.collins.dev outlines really great suggestions. I would just add that I think you don't necessarily need to do both allowing more hits and improving the player movement. Either one on it's own would make things much more palatable. Even reducing the player's collision box width by like 10% would help the area I was having trouble with.
All that being said, there's still a lot of fun in there. Kudos!