Well, first off, let me congratulate you for finishing your first work! It looks really good, and since I think you already know the basics for this gig, I think what you need is sort of a specific lessons rather than just general teachings.
Am, like for fight scenes, try to get yourself some animation guide to understand the way you should portray movement in order to create a powerful image that pretty much captures a whole move on its own. Also, if you find yourself struggling with fight moves and that kind of stuff perhaps you should watch martial arts exhibitions videos, for example, and try to study some basic poses or like the basics stances of fight so your characters would look more natural.
As for buildings and machinery, well, my two cents here would be patience, lots and lots of patience. See, unlike organic backgrounds -like the ones you nailed in your one-shot-, mechanism and buildings need of a lot more work, because they need you to understand how an engine works for instance or how the weight of a roof works so the building makes sense. On top of that, that kind of background takes a lot of time considering you'll have to work with rulers and all that jazz, so I think using building pictures as a reference and trying to copy and study them would be a good first step. Like testing the waters and see what you are about to get into.
I don't have much experience myself, but I tried to think what would I do if I were you. I wish you luck in this new adventure and just know that I respect and admire you for being determinate and I think, if you persevere just like you did before, you totally got this!
Best,
Hoshie.
P.S. : Am, I don't know if this would help you, but I use some of the guides and tutorials collected in this sites, hopefully you find something of use too. http://anatoref.tumblr.com & http://anatomicalart.tumblr.com
