In regards to the comment below, i like that the perspective is off and the anatomy is slightly distorted.
It is more important that is is so beautifully painted. I especially like the dark wing.

think this is your best piece but thats mebe just me

It's nice to see some non-portrait work from you! I really like the composition here, it really creates the dynamic effect that I think is one of the stronger points of the piece. It also seems like you have a good perception of the light angles on form, which I think is another highlight of this piece

There are a few flaws though.. First of all, the background. The top area with the skyscrapers and the lower area with the gate don't look like they are from the same scene. This is because they are misaligned - they have different perspectives. The skyscrapers have a perspective point in the middle of the sky, while the perspective of the lower background (and the foreground) is tilted diagonally.
The next issue I notice is the inconsistency in the body angles of the demon figure. The right leg faces towards the background, while the body is angled slightly towards the foreground. The left and right legs also seem to have some inconsistency in relation to one another - I can't imagine the angle of his buttocks from that position. The lighting does a good job masking these mistakes, they aren't really apparent unless you focus just on the body angles.
The left foot of the angel figure is tilted up way too high - also, if you tilt your foot up like that, your toes naturally point upward, while his point forward.

I think the areas you should review are definitely perspective and maybe lower torso anatomy - the hip-leg junction is kind of a tough one to get right, especially in a dynamic position like that.
Here's a resource for you:
http://www.alexhays.com/loomis/
The books I would advise you look over are "Successful drawing", for perspective info, and "Figure drawing". I can guarantee that if you get an ideal understanding of perspective and apply it to your drawings, your drawings will look much more accurate. Please keep at it! :)

beautiful peice , love that colors and overall pituee :)

Again, your color usage and compositional skills are this piece's prime assets. However, her foot is not anatomically correct at all--a glaring fault compared to how spot on the rest of her body is--and I feel like the exaggeration of the demon's stride was a slight bit overdone. That 's what my eye sees.