Do what you did before to reach Miror B. Eventually, you will be back to Miror B.'s Hideout to find…wait, where's Miror B.? More importantly, who's that in his place? Talk to him and you'll find out he's Mirakle B., Miror B.'s "rightful successor." Only thing is, he lacks the afro. He'll challenge you to a battle.
Mirakle B.'s Pokémon are stronger than Miror B.'s were. It's recommended to wait until after you beat Venus to beat this guy. If you come back here after you beat the game, Mirakle B. won't be here. You can come battle him any time before you beat the game.
After you win, you can go all the way back to Mt. Battle for a rematch.