Due to the limited nature or scope of the league battles, I'd like to suggest two small changes.
First, to make each "metal" or level a locked-in achievement. That is, once you achieve Silver (for example), you can't go back to Bronze. You can slide up and down between Silver I, II, and III, but you can't slide back down to Bronze I (or lower).
Second, to give a few more options for each round. Limiting it to three randomly chosen opponents that can only be refreshed free once a day, or every 2 hours with a video, often means I've gone days in a row with no viable opponents. This also means I'm reluctant to spend donuts here because a "refresh refresher" often doesn't help much. I'd suggest providing five opponents instead of three at a time, and/or reducing the cooldown for video watching.
These two issues make it very difficult to climb higher in the ranks and to stay at a particular level.