Cute and polished game. When I first started, I encountered a couple bugs:
- I could dash before unlocking it (including through slime blocks).
- Where the game thought the character was seemed to be different from where he actually was. The camera would transition between rooms in the wrong places, and even when I could see what I was doing, I'd randomly take damage when there was nothing nearby. The level collision still worked, however. Do you have separate boxes for the character's level collision and enemy collision? Perhaps they got out of sync.
After starting a new game, however, the issues seemed to go away.
I echo what other people have said about the attack range being too small. A small range could work, but it just doesn't mesh well, IMO, with the boss's size and speed.
I hope you continue this, I definitely enjoyed it. If you do, maybe you could experiment with more slime-based powers, like collapsing into a puddle to avoid enemies and go under low ceilings.