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Pokemon Sword and Shield Gym Leaders
Gym #1 Turffield Gym - Leader Milo
Recommended: Fire, Flying
Rewards: Grass Badge, Grass Uniform, TM10 - Magical Leaf
Moves: Magical Leaf (Grass), Round (Normal), Rapid Spin (Normal)
Moves: Magical Leaf (Grass), Leafage (Grass), Round (Normal)
STRATEGY: Gym Leader Milo only has two Pokemon: both Grass-type. That means there's nothing to worry about in terms of odd type matchups. Any Fire, Flying, Poison, or Bug moves should do the trick. He'll Dynamax his Eldegoss, so it's a good idea to follow suit and Dynamax your Pokemon as well. After winning this battle, Pokemon up to level 25 can be captured.
Gym #2 Hulbury Gym - Leader Nessa
Recommended: Grass, Electric
Rewards: Water Badge, Water Uniform, TM36 - Whirlpool
Moves: Water Pulse (Water), Whirlpool (Water), Horn Attack (Normal), Agility (Psychic)
Moves: Aqua Jet (Water), Bite (Dark), Whirlpool (Water), Fury Attack (Normal)
Type: Water, Rock
Moves: Razor Shell (Water), Water Gun (Water), Bite (Dark), Headbutt (Normal)
STRATEGY: Gym Leader Nessa has three Water-type Pokemon. A strong Grass-type with some STAB moves will serve you very well, as the Water/Rock typing of Drednaw makes Grass moves 4x effective. Electric-type is another good choice. You'll likely out-level Nessa at this point, so the battle shouldn't be too challenging. She'll Dynamax her Drednaw, so make sure to Dynamax your Pokemon at the same time. With Water Badge in hand, Pokemon up to level 30 can be captured.
Gym #3 Motostoke Gym - Leader Kabu
Recommended: Rock, Water
Rewards: Fire Badge, Fire Uniform, TM38 - Will-O-Wisp
Moves: Will-O-Wisp (Fire), Fire Spin (Fire), Quick Attack (Normal), Ember (Fire)
Moves: Will-O-Wisp (Fire), Flame Wheel (Fire), Bite (Dark), Agility (Psychic)
Type: Fire, Bug
Moves: Flame Wheel (Fire), Coil (Poison), Bug Bite (Bug), Smokescreen (Normal)
STRATEGY: Gym Leader Kabu is a Fire-type specialist with three Pokemon. Rock-type moves are excellent for this battle, as they'll be 4x effective against his star Pokemon, Centiskorch. Water-types are a solid choice as well. His Centiskorch will get Dynamaxed the first turn it's out, so Dynamax your Pokemon as well. The Fire Badge will ensure that you can catch Pokemon up to level 35.
Gym #4 Stow-on-Side Gym - Leader Bea
Recommended: Fairy, Flying
Rewards: Fighting Badge, Fighting Uniform, TM42 - Revenge
Moves: Triple Kick (Fighting), Quick Attack (Normal), Counter (Fighting), Revenge (Fighting)
Type: Fighting, Dark
Moves: Bullet Punch (Steel), Circle Throw (Fighting), Night Slash (Dark), Work Up (Normal)
#3 Galarian Sirfetch'd
Moves: Detect (Fighting), Swords Dance (Normal), Brutal Swing (Dark), Revenge (Fighting)
Moves: Scary Face (Normal), Revenge (Fighting), Knock Off (Dark), Strength (Normal)
STRATEGY: In Sword version, Bea is the leader of Stow-on-Side Gym. Gym Leader Bea uses Fighting-types and has four Pokemon. A strong Fairy-typee will be your best bet, as it'll be 4x effective against Pangoro and 2x effective against everything else. Flying-type is always a solid choice against Fighting as well. She'll Dynamax her Machamp, so you should do the same to keep up. The Fighting Badge allows you to obtain Pokemon up to level 40.
Gym #4 Stow-on-Side Gym - Leader Allister
Recommended: Ghost, Dark
Rewards: Ghost Badge, Ghost Uniform, TM77 - Hex
Type: Ghost, Ground
Moves: Brutal Swing (Dark), Hex (Ghost), Disable (Normal)
Type: Ghost, Fairy
Moves: Shadow Sneak (Ghost), Baby-Doll Eyes (Fairy), Hone Claws (Dark), Slash (Normal)
#3 Galarian Cursola
Moves: Ancient Power (Dark), Curse (Ghost), Hex (Ghost)
Type: Ghost, Poison
Moves: Venoshock (Poison), Hex (Ghost), Hypnosis (Psychic), Payback (Dark)
STRATEGY: In Shield version, Allister is the leader of Stow-on-Side Gym. Gym Leader Allister uses Ghost-types and has four Pokemon. This is the first gym leader where typing gets a little more tricky. Ghost type moves are the best choice, but not Ghost-type Pokemon. Dark-types are a safe choice as well. He'll Dynamax his Gengar, so you should do the same to keep up. The Ghost Badge allows you to obtain Pokemon up to level 40.
Gym #5 Ballonlea Gym - Leader Opal
Type : Fairy
Recommended: Steel, Ground
Rewards: Fairy Badge, Fairy Uniform, TM87 - Draining Kiss
#1 Galarian Weezing
Type: Poison, Fairy
Moves: Sludge (Poison), Fairy Wind (Fairy), Tackle (Normal)
Type: Steel, Fairy
Moves: Draining Kiss (Fairy), Crunch (Dark), Iron Defense (Steel), Astonish (Ghost)
Type: Fairy, Flying
Moves: Air Slash (Flying), Draining Kiss (Fairy), Ancient Power (Rock)
Moves: Draining Kiss (Fairy), Acid Armor (Poison), Sweet Kiss (Fairy)
STRATEGY: Opal is the leader of Ballonlea Gym, at least for the time being. She is a Fairy-type user with four Pokemon. Because of the dual-typing of several of her Pokemon, it's hard to cover the spread with a single type. The best bet is some combination of Steel and Ground-types. If you want both in one, Galarian Stunfisk. She'll Dynamax her Alcremie, so keep that in mind and follow suit to keep up with her. Throughout the course of the battle, she'll also ask you some questions. If you answer correctly, your stats will be boosted. The questions are:
Q: 'Do you know my nickname?' A: 'The Wizard'
Q: 'What is my favorite color?' A: 'Purple'
Q: 'How old am I?' A: '16 years old'
With Fairy Badge in hand, you can now obtain Pokemon up to level 45.
Gym #6 Circhester Gym - Leader Gordie
Recommended: Water, Ground
Rewards: Rock Badge, Rock Uniform, TM48 - Rock Tomb
Type: Rock, Water
Moves: Shell Smash (Normal), Rock Tomb (Rock), Razor Shell (Water)
Type: Bug, Rock
Moves: Power Split (Psychic), Stone Edge (Rock), Struggle Bug (Bug), Rock Tomb (Rock)
Moves: Stealth Rock (Rock), Rock Tomb (Rock), Body Slam (Normal), Wonder Room (Psychic)
Type: Rock, Fire
Moves: Tar Shot (Rock), Stealth Rock (Rock), Heat Crash (Fire), Rock Tomb (Rock)
STRATEGY: In Sword version, Gordie is the leader of Circhester Gym. Gym Leader Gordie uses Rock-types and has four Pokemon. A combination of Water and Ground-type moves should cover the dual-typing spread of his Pokemon and be 1x effective or better against everything. He'll Dynamax his Coalossal, so you should do the same to keep up. The Rock Badge allows you to obtain Pokemon up to level 50.
Gym #6 Circhester Gym - Leader Melony
Recommended: Rock, Fighting
Rewards: Ice Badge, Ice Uniform, TM27 - Icy Wind
#1 Galarian Frosmoth
Type: Ice, Bug
Moves: Icy Wind (Ice), Feather Dance (Flying), Bug Buzz (Bug), Hail (Ice)
#2 Galarian Darmanitan
Type: Ice, Fire
Moves: Icicle Crash (Ice), Headbutt (Normal), Taunt (Dark), Fire Fang (Fire)
Moves: Amnesia (Psychic), Freeze-Dry (Ice), Hail (Ice), Icy Wind (Ice)
Type: Water, Ice
Moves: Icy Wind (Ice), Ice Beam (Ice), Surf (Water), Sing (Normal)
STRATEGY: In Shield version, Melony is the leader of Circhester Gym. Gym Leader Melony uses Ice-types and has four Pokemon. A Rock-type will be at least super effective against everything, and a Fighting-type is your second best option. She'll Dynamax her Lapras, so you should do the same to keep up. The Ice Badge allows you to obtain Pokemon up to level 50.
Gym #7 Spikemuth Gym - Leader Piers
Recommended: Fairy, Ground
Rewards: Dark Badge, Dark Uniform, TM85 - Snarl
Type: Dark, Fighting
Moves: Fake Out (Normal), Sand Attack (Ground), Brick Break (Fighting), Payback (Dark)
Type: Dark, Psychic
Moves: Night Slash (Dark), Foul Play (Dark), Psycho Cut (Psychic), Payback (Dark)
Type: Poison, Dark
Moves: Sucker Punch Screech Toxic Snarl
Type: Dark, Normal
Moves: Obstruct (Dark), Throat Chop (Dark) Counter (Fighting), Shadow Claw (Ghost)
STRATEGY: The Gym Leader of Spikemuth and brother of Marnie is Piers. Piers is a Dark-type leader who has four Pokemon. Fairy-type is a must-have, but you'll need something like Ground as well to cover Skuntank's Poison/Dark-type. You'll battle Piers in the town of Spikemuth and not in a stadium, so there will be no Dynamaxing in this battle. The Dark Badge will allow you to capture Pokemon up to level 55.
Gym #8 Hammerlocke Gym - Leader Raihan
Recommended: Ice, Ground
Rewards: Dragon Badge, Dragon Uniform, TM99 - Breaking Swipe
Moves: Stealth Rock (Rock), Sand Tomb (Ground), Rock Blast (Rock), Body Press (Fight)
Moves: Protect (Normal), Glare (Normal), Fire Fang (Fire), Earth Power (Ground)
Type: Ground, Dragon
Moves: Thunder Punch (Electric), Breaking Swipe (Dragon), Crunch (Dark), Steel Wing (Steel)
Type: Steel, Dragon
Moves: Breaking Swipe (Dragon), Body Press (Fighting), Stone Edge (Rock), Iron Head (Steel)
STRATEGY: The eighth and final Gym Leader of Galar is the Dragon-type specialist Raihan. The Hammerlocke leader has four Pokemon. Interestingly, only two of his Pokemon are actually Dragon-type. Either way, a combination of Ice and Ground-types should do the trick against him. He will Dynamax his Duraludon, so you should do the same at that time. The Dragon Badge will allow you to capture any Pokemon regardless of level.
P/s: I hope my answer is some help for you. Since this is my first time answering a question, if there are any mistakes, please help me correct them. Thank you very much!