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 second Elite Four Member continues to be Bruno, the same who reamins to specialize in using Fighting Type Pokemon. He will have evolved both of his Onix into Steelix, but the Pokemon you used to defeat Bruno last time around should still do just fine. Flying Types still have the advantage over his Fighting Types (Hitmonchan, Hitmonlee and Machamp), but they won't be able to do much against his Steelix. With this in mind, it would be best to have at least one Flying Type or Psychic Type on your Pokemon Team for Bruno's Fighting Types, as well as a Water Type (or even a Fire Type) to help with Bruno's two Steelix.
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