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Heatran!!
https://twitter.com/PokemonGoApp/status/1075088365513527298
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It's pretty obvious that Groudon is the king, but you don't really need it. Just doesn't seem worth the RC investment to me if you can plausibly duo it with a best friend and level 30 Rhydons.
Not that Groudon doesn't have other merits, it's a beast in master's league for example. But if you already have one, I don't see much point in powering up a second one just for this boss.