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Thanks to the fact that Sierra is a fixed element in the storyline of the game and not a random online player, her Pokémon lineup choices are easily predicted. To start, Sierra will use Shadow Poliwag as her first Pokémon. After that, her second draw will either be Shadow Sharpedo, Shadow Lapras, or Shadow Exeggutor. The third time around, she could pull out either Swampert, Shiftry, or Shadow Houndoom.
To learn how to defeat Sierra, you must learn how to defeat each of her Pokémon.
Source: https://www.distractify.com/p/how-to-beat-sierra-pokemon-go
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