Cool illustration, you hard work paid off 
General requests for critique like this without context (what is this piece about) or what specifically concerns you are tough because I can get carried away and repaint whole thing, (which would take a lot of time a lot of ppl don't have xD) turn it into something else entirely and in the end still say nothing useful to you and give you only information overload.
A lot of things could be "fixed" and worked on more depending on who you ask, but because I primarily react to colors, light and values in peoples work I did quick op-edits to show how those could be better, mainly on the character:
• I mostly lightened the values of the smoke-cloud to separate it from the character and added more highlights to establish the light source more.
