This was great! I had to get used to the hitboxes at first but I slowly got the hang of it. I would have played for even longer but I couldn't find a replay button so I had to refresh the page and replay the intro each time.
The intro is really cute, though! The little details in the art and the faces on the hearts were adorable. I think my favorite is the picture of the big hero at the top with the glowing purple eyes, looking at his former allies. Your idea for putting a twist on the premise is surprisingly dark and the gameplay goal of not stopping the hero, instead trying to survive as long as possible, goes along well with this. I love media that mixes cute art styles with dark themes so this was right up my alley!
Another thing I really liked is how you put in the ability to choose between 3 different difficulties. As far as I could tell, it only influenced the amount of hearts, but that is still amazing for a 3 hour development time!