Not so sure I can give an unbiased critique, given that I'm the commissioner, but I will say that the piece turned out really good and provide a little more feedback than usual. The references were followed well, especially for a character with so many little details. What was a flat texture in the references (ex: the buckled straps on the legs) has been given some dimension in the drawing, and some slight creative liberty was taken that really adds something nice beyond the references (e.g. the fire coming from the cracks in the hammer, the glowing eyes).
As for the more technical stuff, I think the piece is well framed and the pose is dynamic without being overly complicated--you can see she's just flown up and raised her hammer based on her hair and the fire trail. The use of the paint splatter looking texture on the wings makes them stand out from the clean cell shading on the rest of the figure. I will say that the tattoos on the body are a little tough to make out, but I'd wager that's more due to the busy character design than the skill of the artist.
Anyway, I'm super happy with the piece. Well done!