How to beat Sierra in Pokémon GO is needed information for those that have collected six Mysterious Components. Once you do so, you can create a Rocket Radars which can find Team GO Rocket Leaders. One of those leaders are Sierra.
You can find them in hideouts. They resemble regular PokeStops until you get within range of them, at which point the Team GO Rocket Leader will appear next to the PokeStop.
Here is a list of the possible Pokémon you will face, and which counter Pokémon to use.
How to Beat Sierra in Pokémon GO: Pokémon 1 Sneasel
How to Beat Sierra in Pokémon GO: Pokémon 2 Hypno, Lapras, Sableye
Hypno – Giratina (Origin Forme), Darkrai, Tyranitar, Mewtwo (w/ Shadow Ball), Metagross
Lapras – Machamp, Magnezone, Raikou, Metagross
Sableye – Gardevoir, Togekiss, Granbull
How to Beat Sierra in Pokémon GO: Pokémon 3 Alakazam, Houndoom, Gardevoir
Alakazam – Giratina (Origin Form), Darkrai, Tyranitar, Mewtwo (w/ Shadow Ball), Metagross
Houndoom – Machamp, Rampardos, Tyranitar, Groudon, Kyogre
Gardevoir – Metagross, Dialga, Giratina (Origin Form), Mewtwo (w/ Shadow Ball), Roserade (w/ poison-type attacks)