vs. Champion Rematch
After beating the game the first time and then returning to the Pokemon League to beat the Elite Four, you'll be able to take on the new and improved Champion Iris. Her Pokemon team remains the same, but her levels are much higher than they were the first time you fought her.
Her levels aren't the only thing that has improved, though; all of her Pokemon possess much stronger, more diverse moves, making it a difficult fight! Ice- and Dragon-type attacks are a good bet, but Fighting-type attacks are also effective against a lot of her team.