Very clever. I love the environments and art, I love the mechanics and puzzles, I LOVE THIS GAME.
One exploit I found though was to hang out in the water level and jump between the screen with the long platforming and the screen with the sea shell. whenever I entered the platforming room, a new medusa head would spawn low enough for me to kill it and get dat cash, and then instantly turn around and go back into the sea shell room. doing this for a couple minutes can get you a couple cheap levels, and make the final boss super easy.
Also, can I just say how awesome the bossfight with the Devil was? Bullcrap platforming, but awesome nonetheless. I like!