After the Elite Four You can also catch Mewtwo, who is proportionally difficult to Zygarde (it's easier to get to him, but the battle is long). Head to the Pokemon Village outside of Snowbelle City, and make Your way to a cave in the north-west. Once You get inside the psychic legendary will battle. Repeat what You did with Zygarde, weaken it and shower him in Poke balls. Metwo may take some time, as he constantly uses Recover, to regain HP. After a while, he will run out of PP, and You should be able to weaken him effectively. I strongly recommend using an Electric type (moves like Thunder Wave really do the trick). Once the HP is in the red zone, and after a few balls, he should be Yours.