Instructions
Switch your team to six high leveled Pokemon with various types and attacks. You should have no more than two of a given type, as the Elite Four have varied attacking capabilities. The first trainer, Aaron, has Pokemon of the bug type, so bring along a strong Fire type. His first Pokemon is Dustox, so hammer it out with flying, fire, or psychic types. Next comes Heracross, which is incredibly susceptible to flying attacks. His Vespiquen and Beautifly are incredibly weak against Rock. His final creature, Drapion, is a challenge, being weak to Ground and not much else.
Challenge the second of the Elite Four, Bertha. Her team consists of mostly Ground types with a few Rock types, too. She starts off with Quagsire, which is immensely weak against Grass attacks. Defeat her following Hippowdon with a strong Grass or Ice type. Grass is the way to go, as her Sudowoodo and the following Whiscash can't withstand it. When it seems like the Grass party ends, she'll bring out Golem. Hammer away with more Grass moves to bring him down.
Defeat Flint by capitalizing on Pokemon types. Teach a water type Ice Beam or other powerful Ice attacks, as only Water moves won't defeat Flint. His Rapidash, Infernape, and Steelix all can have their flames doused with a strong Water move, but his final Drifblim needs an Ice Beam or two to knock it out of the park. As an added oddity, Flint employs a Lopunny, a Normal type Pokemon. Take it out with a few Fighting type moves.
Defeat Lucian by having a strong Ghost and Dark type. Having a strong Dark type with Ghost moves can also work. Lucian team is comprised of: Mr. Mime, Girafarig, Alakazam, which can't handle Dark attacks. Medicham is a dual Psychic and Fight type, meaning Dark attacks won't do the same damage. Overcome this with a strong Ghost move. Lucian will end the battle with Bronzong, a Steel and Psychic type. The odd one out, Brozong will be able to handle Ghost and Dark, so use a strong Fire attack to take him out.
Get ready for the Elite Four's hidden fifth, the reigning champion. The champion is just plain mean and will open with Spiritomb, one of the few Pokemon that has no type disadvantage. You simply need to hammer out the strongest attacks from the strongest Pokemon you have and hope. Her second, Garchomp, is laughably weak against Ice. The third, Gastrodon will keel over with a Grass attack or two. the next two, Milotic and Roserade, can be handles with a few Flying attacks. The final one, Lucario is weak against Ground and Fire attacks. Win the battle and be the next champion.