A nicely made Metroidvania. Loses points for not quite sufficiently explaining one of the key points in the ending, as well as a door that was so undistinguished from it's surroundings that I had to consult a walkthrough. (It was the one between Kuriiri and Skim The Sky, if you're wondering.)
Four out of five stars, and I hope to see more from this developer.
Decent game, but the second boss on hard mode could use a bit more explanation, as it appears to be invincible. In normal mode it can be killed simply by shooting it, but in hard it only seems to take damage if you can lure it into a wall during it's third attack pattern. Also, be sure to get 100% completion, as the achievements each come with a permanent powerup. Achievement 2, in particular, makes Hard mode much easier.
Actually, I could suddenly kill box enemies some time before elemental enemy immunity suddenly disappeared, so either a feature or two glitches in a row.
I... I killed the cat. Good grief. Also, I no longer need to use matching spells
on elemental thingies, and like a previous commenter I can now harm the
block-thingies. Probably a glitch connected to your level, but helps with the
final boss a lot.
@Doomkid: If you prefer challenge to plot/graphics, Thing-Thing 2 (specifically Zeus) was quite hard, and often counter intuitive (kill people to open doors?), anything under 15 or so kills counted as PACIFIST rank.
 "Legend of Kalevala"){kind=icon}
 "Blasting Agent"){kind=icon}
 "Don't Escape"){kind=icon}
 "Arzea"){kind=icon}
 "Thing-Thing 4"){kind=icon}