Instructions
Travel to Pokemon League.
Talk to the person at the counter to enter the Pokemon League Challenge and face the Elite Four.
Defeat the Elite Four trainers Aaron, Bertha, Flint and Lucian. Aaron uses Bug Type Pokemon, so you should use Psychic Type Pokemon. Since Bertha uses Ground Type, use Grass or Water Type. Flint uses Fire Pokemon, so Water Type Pokemon are your best bet for victory. Lucian utilizes Psychic Pokemon, so unless you form a Ghost Type team, he will have the advantage.