I often shy from critiquing work that I haven't commissioned, but as you've requested one, I can oblige.
Your overall use of color and thick roughness of your hand causes the work to wander slightly towards impressionism. This isn't necessarily a bad thing, it's a stylistic choice. But for example if you were to submit this against someone who submitted a photorealistic piece that was entirely digital, your work would look blurry next to his. As a standalone, or as a stylistic piece, it works fine, though.
As far as composition, two things that would I would alter are the rather bored looking poses, and the dragon's turtle-like back. The archers look like mannequins who were set up in place with bows drawn, rather than dynamic, living people who are fighting for their lives. Smaug's neck looks so short as to be almost non-existent, and his back is so thick that he looks almost turtle-like.
Ultimately, though, you have a superior sense of coloration where Smaug's flaming breath is concerned. I have found in working with dozens of artists that achieving a glowing, radiant effect is not a common skill. That fire so dominates the work that I get a real sense of the light and heat boiling out of that dragon's mouth. Your mastery of this effect is so strong that the other problems tend to melt into the background unless the viewer is actively searching for things to nitpick about.