After concluding the Post Game Story in the Sevii Islands, you can return to Indigo Plateau to rematch the Pokemon League. The Pokemon League will continue to be comprised of the Elite Four and Kanto Region Champion, five trainers you must still defeat consecutively in order to defeat the Pokemon League. This time around, however, the members of the Pokemon League have gotten all the stronger, even switching out at least one of the Pokemon they previously had with one found in either the Johto or Hoenn Region.
The third Elite Four Member continues to be Agatha, the same who remains to specialize in using Ghost Type Pokemon. She will have evolved her Golbat into Crobat and switched her Haunter out for the likes of Misdreavus, but the Pokemon you used to defeat Agatha last time around should still do just fine. Four out of five of Agatha's Pokemon (all outside of Misdreavus) are part Poison Types, giving them a huge weakness against Psychic Type Attacks. With this in mind, it would actually be best to have at least one Psychic Type on your Pokemon Team (as well as a few Pokemon with a Psychic Type Attack) in order to defeat Agatha.
Click Here To Learn How To Easily Defeat Elite Four Lance In Rematch!