There are some major problems here but also some major potential. The first thing I noticed was a style of minimalism which sometimes helped the game and sometimes hurt it. The silhouette platforms were good. The menu buttons were good. The voice acting was good (though repetitive). The lack of boss music was bad. The lack of animation for the enemies was really bad. The lack of any sort of AI apart from the boss was very bad. The lack of a menu background, bad. The general lack of things to do in the game, very bad. The fact that the difficulty only affects your health, also bad. Even when you have a crouch feature, you only use it a few times throughout the entire game.
Overall the game showed signs of polish, but just as many signs of a lack of polish. The textures in the background were well-done, the character sprite is well-done, and the boss, interestingly enough, has well-drawn hands and a very sloppy looking body. I like how the lava had a delayed reaction in response to the boss' pounds, and how the screen shook with each attack. I see that you tried to spice things up with a flight mechanic in level 4, but it's not enough. You need to incorporate stuff like special abilities, collectibles, or features like climbing, sliding, or grabbing. This game is lacking in content and it hurts it a lot. Overall the gameplay becomes very formulaic due to the fact that the enemies don't even respond to the player. It also makes the player feel disengaged, which could have been rectified by implementing some kind of plotline.
I also want to mention that you really need to do something about that boss battle. I actually liked the physics - it's neat how the player moves faster while jumping - but I found many times that the player was sometimes fast enough to out-walk the boss' attacks, and was sometimes too slow. I noticed the player tended to die from this while walking outward from the center more often than if the player was walking toward the boss, but it wasn't a sure thing. The mechanics were at points sloppy, having once been pushed back into a wave of lava by a hand, or other times falling straight through the ground because I didn't jump quite high enough (at least stop the player from moving forward, show them that they've hit a wall).
Anyway I just want to reiterate that the boss fight is hell if you're going for achievement 05. It's frustrating and fickle and I feel like you didn't have enough people beta test the project. It would have been nice if the boss had more attacks, but more importantly the delay between the attacks needs to be greater, or at least more consistent.