After defeating the Elite Four and becoming the newest Kanto Region Champion, you can rematch the Pokemon League whenever you would like. When facing the Elite Four again, each member will use the same Pokemon as before (but at higher levels), as well as adding one Alolan Form Pokemon to their team that matches the type of Pokemon they specialize in using. You will continue to earn loads of Experience and Money when you rematch and defeat the Elite Four.
When it comes to Agatha, the third member of the Elite Four, she will still specialize in using Ghost Type Pokemon and Poison Type Pokemon, continuing to battle with Arbok, Gengar, Golbat, Weezing and a second Gengar. However, she will also use a Marowak in its Alolan Form when battled. This time around, all of her Pokemon will be at Level 63, with the exception of her final Gengar that will be at Level 64.
Just like your first battle against her, Ground Type Pokemon actually work best against Agatha's Pokemon Team. This is because almost all of her Pokemon are also part Poison Type, meaning they are especially weak to Ground Type Attacks. Even her Alolan Marowak, which is part Fire Type, is weak to Ground Type Attacks. But don't dismiss also using a Psychic Type or two when battling Agatha, since all but one of her Pokemon are also weak to Psychic Type Attacks. Finally, make sure the Pokemon you do use against Agatha are at Level 65 or greater, as this will allow you to match up well against her and the rest of the Elite Four.