Back in April, the various members and leaders of Team GO Rocket were absent from Pokemon GO for quite some time. At this time, however, they've all returned and remain just as strong as ever. This includes Sierra, one of the challenging leaders of the team.
After getting a Rocket Radar, players can find Sierra in Pokemon GO either in one of the Rocket Balloons or having taken over one of the various Pokestops. Like all of the Team GO Rocket leaders, players can't choose which leader to encounter with Sierra being just one of the three possible leaders.
When fighting Sierra, players should note that while she will only use three Pokemon like with any other trainer battle, she actually has seven Pokemon in total. This is because both her second and third party members have three different Pokemon that can be sent out. There is no way to tell what Pokemon Sierra will send out unless players rematch Sierra after losing to her within the same encounter.
Sierra's First Pokemon - Carvanha
No matter what, players will be facing Carvanha as their first opponent against Sierra. This Pokemon is a dual Water and Dark-type, making it weak to Fairy, Bug, Electric, Grass, and Fighting-type moves. It however also resists Ghost, Steel, Water, Ice, Dark, Fire, and Psychic-type moves.
There are quite a few Pokemon that can deal with Carvanha quickly. Some of the best ones are:
Therian Thundurus - Thunder Shock and Thunderbolt
Machamp - Counter and Dynamic Punch
Raikou - Volt Switch and Wild Charge
Sirfetch'd - Counter and Close Combat
Zapdos - Thunder Shock and Thunderbolt
Sierra's Second Pokemon - Hippowdon, Porygon-Z, or Mismagius
As previously mentioned, there are three different Pokemon that Sierra can use as her second party member. The first of these is Hippowdon, a pure Ground-type Pokemon. Its typing makes it weak to Grass, Ice, and Water-type moves while also resisting damage from Rock, Poison, and Electric-type moves. The best counters against it are:
Galarian Darmanitan - Ice Fang and Avalanche
Kingler - Bubble and Crabhammer
Kyogre - Waterfall and Hydro Pump
Roserade - Razor Leaf and Solar Beam
Black Kyurem - Dragon Tail and Blizzard
Another possibility is Sierra's Porygon-Z, a pure Normal-type Pokemon. It is only weak to Fighting-type moves while also only resisting Ghost-type moves. Some of the best Pokemon to deal with it quickly are:
Lucario - Counter and Aura Sphere
Conkeldurr - Counter and Dynamic Punch
Breloom - Counter and Dynamic Punch
Machamp - Counter and Dynamic Punch
Blaziken - Counter and Focus Blast
The final Pokemon that Sierra could use in her second slot is Mismagius, a pure Ghost-type. This makes it weak to only Dark and Ghost-type moves while resisting Bug, Poison, Fighting, and Normal-type moves. Its best counters are:
Gengar - Lick and Shadow Ball
Absol - Snarl and Payback
Chandelure - Incinerate and Shadow Ball
Mismagius - Hex and Shadow Ball
Sharpedo - Bite and Crunch
Sierra's Final Pokemon - Houndoom, Flygon, and Walrein
Like before, there are three different Pokemon that will serve as the final part of the battle with Sierra. The first of these is Houndoom, a dual Dark and Fire-type Pokemon that is weak to Fighting, Ground, Rock, and Water-type moves. It may resist damage from Steel, Ghost, Grass, Ice, Dark, Fire, and Psychic-type moves, but players can still counter it with:
Lucario - Counter and Aura Sphere
Rampardos - Smack Down and Rock Slide
Kingler - Bubble and Crabhammer
Therian Landorus - Rock Throw and Earth Power
Conkeldurr - Counter and Dynamic Punch
Next is Sierra's Flygon, a dual Ground and Dragon-type. It is weak to Ice, Fairy, and Dragon-type moves, but it also resists Rock, Poison, Fire, and Electric-type moves. Some of the best Pokemon for players to use against it are:
Galarian Darmanitan - Ice Fang and Avalanche
Black Kyurem - Dragon Tail and Blizzard
Mamoswine - Powder Snow and Avalanche
Weavile - Ice Shard and Avalanche
Glaceon - Frost Breath and Avalanche
Finally, the last of Sierra's Pokemon is Walrein, a dual Ice and Water-type Pokemon. It boasts more weaknesses than resistances with it being weak to Fighting, Rock, Electric, and Grass-type moves while only resisting Water and Ice-type moves. Because of this, the best Pokemon for this job are:
Therian Thundurus - Thunder Shock and Thunderbolt
Roserade - Bullet Seed and Grass Knot
Therian Landorus - Rock Throw and Superpower
Leavanny - Razor Leaf and Leaf Storm
Therian Tornadus - Gust and Grass Knot
Pokemon GO is available on Android and iOS.
Source: Pokemon GO Info