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I don't know if Serebii has updated it yet, but a few weeks ago his page on locations to evolve particular Pokemon mentioned that you can only evolve Nosepass/Magneton after Primal Groudon/Kyogre when New Mauville becomes accesible but you can just bring Nosepass/Magneton to the entrance and run around in front of the locked door to find some wild Voltorbs and Magnemite that can help you level up and evolve.
