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So it's safe to assume that he's stronger than all the gym leaders in Kanto, Johto, Hoenn, and Sinnoh as well as no doubt being the strongest Kanto(Hoenn) Frontier Brain. Did Paul manage to beat him eventually?
But most importantly, 2 new regis are coming with the SwSh DLC and if Regigigas awakened again we could get a full regi team Brandon. Ash never battled all the regis on one team so could he beat Brandon if he only used Regis?