The Elite Four battle is after the victory street and all the gym battles. Your first battle Sidney who specializes in dark types. Then Phoebe with her ghost type Pokémon followed by Glacier and her ice type Pokémon. Drake is the last elite and specializes in dragon types. Each elite`s house opens after you go close enough. At the end, you battle champion Steven and his Mega Metagross. After this, the elite four want a rematch. At the rematch, the are much more powerful and each have a Mega Evolution from Steven. You battle him again and this time there`s no rematch.