The overall picture looks very nice, the only thing I would change is the girl. She's holding a gun, but her expression doesn't say badass or threatening to me, so much that I noticed the gun in her hands after looking at the picture for the 2nd of third time. I took the pose and it was very hard to maintain, let alone trying to walk like that. Lastly, I feel the composition would be a bit stronger if she wasn't at bit a the back like she is.

That being said, your work on the background is lovely and wish you the very best of luck!

You should make different base greys for each thing to add further contrast in terms of the materials that make them up. For example, the tree bark should be much darker, the grass and leaves should be lighter, and the clothes can be various shades of grey. This will make everything stand out more, so that there is more than just your line art differentiating each part from the other.