I don't like the fire on the fist too much. It comes out too fake, if you would hit a wooden stick that is on fire against something you would at least see some fire sparks around it, not just solid flames. Also moving fire will give it a clear direction, I'm kinda missing that here. I do love what you did with the sack, and I guess the sparks on the left side somewhat resemble the universe? I just don't know why the right side is completely blank. And I like what you did with his arm, perhaps that could be expanded towards the full body?
Good job though