Hmm.. I've never played battle levels online, but some general rules that apply to just about anything... will probably apply to battle levels as well.

The most important thing I can think of is contrast. You want your battle levels to be easily navigatable; online games are obviously quite hectic. To be able to easily navigate through a level without first having to look where you can go in the first place, it helps if the 4th layer is easily discerned from the background.
Basically, if the ground is white and the background a black horizon with stars, any half-skilled player should be able to run quickly through it while being able to concentrate on other players instead of the surrounding area.