After playing through this 4 or 5 times, I've decided to write up a review:
Screen 1: Upwards diagonal skip that is more than worth doing. Someone could do the easy jump after it, hit the save, then come back and do the skip with minimal cost of time on death. (Note: I realize the save isn't there on hard mode, but it's still worth it to do the skip on hard, thus making the second half of the segment pointless.) The last save before the portal is far harder than the rest of the screen, on any difficulty because of the two end jumps. (Note: Those jumps (or very similar jumps) appear quite often. They are present in two of the first three screens. Dying often to the same type of jump can feel very grindy and unmotivating, as well as lacking creativity. Any way you can spice up a couple of those jumps into something new and creative would be much appreciated amongst everyone.)
Screen 2: When I first played this, I went head first into the shit brown block with grass on it in the water because I didn't see it. I suggest using either a spike or changing the opacity of one of the object's. The corner skip is appropriate difficulty relative to the segment it skips. Other than that, I enjoyed this screen a lot and felt like there was a good variety of jumps, as well as consistent difficulty.
Screen 3: The difficulty spikes way up on this screen. You bring back the last two jumps from the first screen and put a carbon copy in this screen, as well as two very similar jumps to end this screen just like in screen 1. Did you want the minispikes to show inside the blocks on the final segment? If not, then make the depth of the spikes more than that of the blocks. This will cause the body of the spike to disappear, leaving just the tip. The last save is far harder than the rest because of the final jump.
Screen 4: Felt a lot easier than screen 3. Was the apple in the top right corner supposed to fly off when the trigger was touched? It didn't for me, when all the others did. I liked this screen because of the variance in platforming, as well as the incorporation of triggers in a way that didn't involve death. Probably my favourite screen in the game.
Boss: Felt like a grind, not only because of the long i-frames but because of the lack of a health bar. Might I suggest more health with no i-frames or a shorter length of i-frames? That way heavy mashers can go for quick kills and make the boss a bit more exciting. The boss difficulty-wise felt appropriate for the difficulty of the game, maybe a touch harder than it should be. A suggestion would be one more type of attack.
All in all, not a bad effort for a few days' worth of work and your first fangame. Keep it up, and make sure to listen to constructive criticism. If you don't have any testers, make sure in the future you have testers of varying skill levels to make sure the difficulty curve stays well-balanced. Good job!