The dash is great and the bosses are quite fun. The controls were pretty tight and the UI wasn't cluttered. The big issue was the visuals, because they were stretched. A quick tip for pixel art is to always scale the sprite equally. If you don't, it'll look all weird and stretched like it sometimes did in this. The music was a bit annoying after a while, so one way to improve could also be to add some variations in music in boss battles, different areas, etc...
Overall, the game has great gameplay, but the visuals and audio could be improved on a bit!