For bigger scenes like this, I'd recommend blocking it out in basic colors first. Don't worry about the details or AA. Just focus on the composition first, then colors and basic lighting. Once you're happy with all of this, you can start polishing. Sketching out the scene helps prevent running into composition related problems later on and having to redo details. Additionally, if you start drawing at pixel-level detail, it can be discouraging because 99% of your piece is blank space.
Sometimes it helps motivate me if I polish my favorite part of the scene first, because if it comes out looking good I want to buckle down and finish the rest of it.
