The Oceanview Power Plant has become the home to electric type Pokémon. Though abandoned, the machinery still functions. You will encounter trainers on the way to the top, although they are optional.
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On the third floor you will find the gym leader and the Pikachu you previously defeated. Defeat the Pikachu to unlock the gym. Once you defeat the Tunod League, you obtain access to the rooftop where you can catch Zekrom.