So, I managed to get stuck at the bottom of one of the tutorial screens. Twice.
Please don't ask how I got there, or why I fell there again despite knowing about the bug; I'm not very good at platformers. None of the controls seemed to work once I got down there, so I had to reload the web page. I'm not sure why I got stuck there—maybe because there wasn't a spot on the next screen down for me to spawn?—but it seems like a bug worth trying to fix.
Maybe take this with a grain of salt, considering that I'm self-described as "not very good at platformers," but I'd have liked it if there was another checkpoint—maybe before the spike room? And the hands shouldn't flash red when attacking; it took me too long to realize that the flashes were unrelated to me "damaging" them. On the other hand, the fact that the gems you're supposed to attack don't respawn when you fall off the bottom of the screen made it very easy for even me to cheese the boss fight—I just jumped down, threw my sword, and repeated four times (a skull got in the way once).
Aside from difficulty curve stuff, it's a good little game. The movement mechanics are neat and could probably be turned into a more complete game; the combat is perfectly sufficient for a platformer and works with those mechanics. The pixel art and sound effects are good, too, though the big slime just being an upscaled slime with pixels the size of the main character...games where pixels on different sprites are different sizes are a pet peeve of mine, and that was an extreme example of that.