Making battlebacks is fun! (if a little frustrating) It's important to keep in mind where your enemies are going to be, because you don't want the details too cluttered in those areas, or you're just going to end up swallowing your enemies in those details. It may end up that the image looks a bit strange on its own by doing this, but since it is designed to exist with enemies on top, you should be more concerned with that. Not only is watching out for cluttered details making the battlers hard to see important, so is making sure the colours are handled in a way that allows the enemy graphic to pop out. <--- It's gonna take some trial and error to figure out what works best in that sense.
As with drawing any sort of backgrounds, look at a lot of scenery photography and keep in mind the
Rule of Thirds. But even more importantly than the rule of thirds, is just being aware of your battle layout. It would be a shame if you put your most detailed elements in a spot that ends up being completely covered by some sort of HUD or menu.
If you are trying to match the RTP style, then looking at it carefully and seeing how it works would be good; but if you're wanting to go completely custom, then play with styles until you find something that works well for you!

((Also, I find it helpful to work at 2x the size of what you need in game. When you shrink the graphic, little errors or places that were a bit messy can be hidden

))