The intro is too condensed and fast-paced. I had to watch it twice to catch what it was trying to show. Also suddenly taking control during the tutorial make the player fall into a pit makes it feel more like a glitch or like your mechanics aren't very good. Even if the player learns better afterward, that's not a good first impression. Having an invisible wall above the wood is similarly not a good idea.
Your visual design is great when it comes to the individual assets, but their use in the level design, especially the way the ground is layered with the not-ground, is visually confusing. Related, the bars really should be split into multiple layers so that the character can actually go through them. Otherwise it's unclear the first time whether they're an obstacle or a mistake in development.
The lava form's blast needs more impact. It looks like a puff of air.
While I was playing the lava form's movement speed became stuck at the rock form's movement speed, preventing me from clearing jumps I'd already cleared. I think it occurred because of getting hit by water while using the blast, but it's hard to tell. I played through to the spot with the button to turn on a flame required to get through a gate and then could make the jump afterward. Then I played through again (to figure out why such a well-made game was leaving me bored) until the part with the large pool with sprays, one of which goes up further into a secret area. The bug definitely occurred the second time, but I'm not sure if it occurred the first time.
I'm having trouble figuring out why, but your challenges seem bland. I get less the feeling of figuring things out and more the feeling as if I'm just going with whatever happens.