The girl is drawn great too. Uh, tentacles seem like an afterthought though.
A good rule of thumb is to keep backgrounds pretty simple. Most other things it's sort of like anything else you draw. So construction, value, etc. all still in there.
You want to give priority to the foreground and w/e is in it though, so there's added challenge. Look up value contrasts, or contrapposto if you're running in expert mode.
Comp is a little unbalanced here. Would have benefited from more thumbnailing, but no big loss. I guess you're breaking out of the single detailed asset into something that uses the full frame more. That's great, but you can push it further. Look up the terms I listed above, also framing, and rule of thirds. Sure there's more I missed.