Wow, amazing piece!
There is not much critique from me, but I do have some points you might want to work on.
The whole piece shows you have put much effort in it. Every pixel is placed carefully, but the sky (not the clouds) looks a bit out of place. Maybe you can improve this by filling up the whole sky with new lighter clouds, you can mix the color of the sky and the color of the current clouds so it looks like the new clouds will be further away. The new cloud thing is just an idea, but I really think the piece will improve if the brown sky will be filled up.
There is one thing that looks very strange in this piece, the shading on the back of the monster. The monsters back is so light, but the place is clouded so I can't see where the light is coming from. You should replace the white by a light pink.
You can also add lightning to the background, this will make the light back of the monster make sense. You can also shade its back blue when you use the lightning, I think it will have a nice effect on it.