Beautiful shading, as always Mex. It almost looks as if it was 3-dimesional, looking at the 'depth' of the lines on Groudon's hands and head. It is truly spectacular on what you can do with only a pencil.
The head is totally okay, but what troubles me is the expression on Groudon's face. It's mouth is open, but I don't seem to see any expression or energy soming from it, the jaw doesn't look fully open to express much. But I understand that you referenced from the Colosseum box cover, where Groudon's mouth is open.
Another thing is that it's arms look too static, or very still, like a very detailed Groudon doll. It makes it look expressionless, just sticking out it's arms like that (it's not to offend, just pointing it out). I suggest that you should make a sketch of it's pose before starting on the details.
I also see disproportion on it's arms, It looks somewhat smaller compare with the body, and the claws look too short. I recommend that you should use more than one image for reference, like the official artwork such as
this from the Bulbagarden Archives.
And yes, I would like too see this colorised.