While you've been traveling all around the Kanto region, the various Gym Leaders that you've defeated have been secretly training to keep up with you. Once you've conquered the Pokémon League, you can rematch the Gym Leaders at their original locations. You can also battle each of them again when a new day starts.
Gym Leader Brock
As the very first Gym Leader, Brock has received the largest upgrade. Still, he's pretty easy if you have reliable Water and Grass-types.
First up is Brock's trusty Onix. It can use Rock Slide and Earthquake for STAB, Iron Tail to damage Fairies etc., and Stealth Rock to deal damage to Pokémon that you switch in. Water and Grass are ideal, since they deal quadruple damage, and you want to quickly KO it before it uses Stealth Rock.
Next, he may send out Omastar, one of his three fossil Pokémon. This one can use Hydro Pump for STAB, Blizzard to hurt Grass-types, Seismic Toss to deal 56 damage, and Reflect to weak your physical attacks. If you have one, now's the time to sick your Grass-types on it. Electric-types are also good here.
Golem is Brock's "main" Pokémon, although it's not much stronger than his rest. This one can use Rock Slide and Earthquake for STAB, Fire Punch to hurt Grass-types, and Thunder Punch to hurt Water-types. Still, Grass and Water will rule surpreme here.
Aerodactyl is another fossil Pokémon--and quite possibly the most dangerous one. It can use Rock Slide for STAB and to potentially cause flinch, Earthquake to pummel Electric-types, Iron Tail, and Crunch. Water-types are the safest choice, so long as they're not weak to Rock. Looking at you Gyarados.
Finally, Kabutops completes the trinity of fossils. This one can use Rock Slide for STAB, Aqua Jet to try and go first, Brick Break to shatter barriers, and X-Scissor. Like Omastar, Grass-types are ideal here because of their outrageous damage. But non-Poison ones should be wary of X-Scissor.
Gym Leader Misty
Similar to Brock, Misty is a significant step up compared to the last time you fought her. As before though, you should have a pretty straightforward time if you have some solid Grass and Electric-types.
First into the swimming pool is Golduck, which Misty must have evolved from her Psyduck. This one can use Scald for STAB and to potentially burn, Ice Beam to hurt Grass-types, Psychic to damage Poison-types, and Light Screen to weaken special attacks. Try and KO it fast with a strong Grass or Electric-type.
After this, Misty may send out Vaporeon. This Water-type Eevee can use Hydro Pump for STAB, Blizzard to wreck Grass-types, Shadow Ball to surprise Psychics and Ghosts, and Quick Attack to finish off weakened foes. Vaporeon is fairly bulky, so be prepared to eat an attack from it.
Once again, Misty has her trademark Starmie. It can use Hydro Pump and Psychic for STAB, Dazzling Gleam to batter Dark-types, and Thunderbolt just in case. Healthy Electric-types are your best bet, unless you have a non-Poison Grass-type in the wings.
Gyarados is Misty's strongest Pokémon and it shouldn't be underestimated. Her Gyarados can use Waterfall for STAB, plus Outrage, Crunch and Iron Tail to hurt a variety of targets. Electric-types are its bane, so try and save one for this very moment. Otherwise you may have a difficult battle.
Finally, we have Dewgong. This one can use Waterfall for STAB, and Iron Tail because... well, that tail has to be good for something, right? Anyway, it's also carrying Aqua Jet and Ice Shard. Grass-types are ideal, but you'll need to anticipate an Ice Shard coming your way. Otherwise, go with Fighting-types.
Gym Leader Lt. Surge
It should come as no shock (sorry for the pun), that Lt. Surge still favours Electric-types. As such, now would be an opportune time to roll out your sturdy Ground-types.
Surge's first Pokémon is Electrode. It can use Thunderbolt for STAB, Foul Play to damage high Attack Pokémon, Mirror Coat to retaliate against special attackers, and Explosion to blow itself up. Obviously, a Ground-type would be the perfect counter. If you're afraid of Explosion, use Protect.
Next, Surge may bring out his Electabuzz. It can use Thunder Punch for STAB, Fire Punch to hurt Grass-types, Ice Punch to hurt Ground-types, and Brick Break to demolish any protective barriers. Despite Ice Punch, Ground-types are the superior choice.
Moving on, we've got Magneton. This one can use Thunder and Flash Cannon for STAB, Tri Attack to potentially inflict a status condition, and Hyper Beam for a sudden burst of damage. Because it's also Steel, Ground-types will do considerable damage. Alternatively, go with Fire or Fighting if you want your Ground-type to take a break.
Jolteon is an Electric-type Eevee that boasts obscene Speed. It can use Thunder for STAB, Shadow Ball to deal neutral damage to most foes, Quick Attack to go first, and Reflect to weaken physical damage. Unless you're using something like Dugtrio, there's a good chance Jolteon will act before you do, so keep that in mind.
Finally, Lt. Surge has his favourite Raichu. It can now use Thunder Punch for STAB, Brick Break to break barriers, Iron Tail to hurt Rock-types, and Quick Attack to sneak in some damage before you do. So long as your Ground-type isn't Rock as well, this should be a piece of cake.
Gym Leader Erika
The gentle Erika has an all-Grass team, so it's a good idea to face her with Fire or Flying-type monsters.
The battle begins with Tangela, the only pure Grass-type here. It can use Power Whip and Mega Drain for STAB--the latter also stealing HP. Plus Sludge Bomb, and Reflect to weaken physical attacks. In addition to Fire and Flying, Poison and Bug are fine choices too.
Afterwards, Erika can send out her Exeggutor. This one carries Solar Beam and Psychic for STAB--the former requiring a turn to charge up. It also knows Sludge Bomb and Hyper Beam. If you're not afraid of Psychic, Bug-types can be very useful here because of their quadruple damage. Otherwise, Dark is a great choice.
Parasect can either be a massive nuisance or gone in the blink of an eye. It can use Leech Life and Solar Beam for STAB--the former steals HP, while the latter requires charging. Plus Brick Break to remove barries, and Spore to guarantee sleep. Fire or Flying-types are highly recommended to one-shot it before it uses Spore.
Vileplume is Erika's choice Pokémon. It can now use Petal Dance and Sludge Bomb for STAB--the former causes confusion to itself after a few turns. Plus Moonblast and Stun Spore. Besides Fire and Flying, Psychic is also a stellar choice.
Finally, Erika has evolved her Weepinbell into Victreebel. Now it can use Power Whip and Poison Jab for physical STAB, Swords Dance to greatly boost its Attack, and Reflect to protect its team from physical attacks. Similar to Vileplume, Fire, Flying and Psychic are the best choices offensively and defensively.
Gym Leader Koga
The ninja master Koga has trained his Poison-types to cause you a shameful defeat. If you want to avoid that scenario, you may want to prepare your best Psychic and Ground-types.
Koga's first Pokémon is Tentacruel, so put away your Ground-type for a moment. This sinister jellyfish can use Hydro Pump and Sludge Bomb for STAB, Mega Drain to steal HP from Ground-types, and Toxic to fatally poison your companions. Psychic and Electric-types are recommended.
For his second Pokémon, Koga may send out Venomoth. It can use Bug Buzz and Sludge Bomb for STAB, Psychic to hurt Fighting and Poison-types, and Mega Drain to overcome Rock and Ground-types. It's easy prey for Psychic, Fire and Flying-types though.
Golbat you should be very familiar with. This one can use Fly to disappear for one turn, before inflicting STAB damage. It can also use U-Turn to deal damage and switch out, Leech Life to drain HP from Psychics, and Crunch for other purposes. Electric or Rock-types should do great here.
Muk is Koga's trump card. It can use Sludge Bomb for STAB, Mega Drain to annoy Ground-types, Dark Pulse to punish Psychic-types, and Moonblast to counter Fighting and Dragon-types. Despite this, Ground and Psychic are recommended, unless you want to wall it with Steel-types, for instance.
Finally, there's Weezing. It can use Sludge Bomb for STAB, Toxic to put a countdown on your Pokémon, Fire Blast to melt Steel-types, and Thunder to surprise Water and Flying-types. Ground and Psychic-types should have no problems here.
Gym Leader Sabrina
Sabrina is as strong as always, although admittedly it hasn't been that long since you last fought her. As such, you should be fairly well-prepared for her. Like the previous time, Dark-types are recommended. But they are rare, so you're welcome to brute force the fight.
First up is Mr. Mime, just like last time. Now it actually has a Fairy-type move, so Dark-types should be benched, unless they're Alolan Muk. It also has Psychic for STAB, plus Reflect and Light Screen to weaken physical and special attacks. If you have one, a Steel-type would be perfect.
Slowbro is slightly less tricky, but you can't afford to let your guard down. It can use Scald and Psychic for STAB--the former can cause a burn. Plus Ice Beam to damage Grass-types, and Shadow Ball for other purposes. Besides Dark-types, Electric-types are worth a shot. Grass-types too if they're unafraid of Psychic.
Hypno can be annoying since it can use Hypnosis to cause sleep, followed by Dream Eater to deal STAB damage, while stealing HP. Of course, if you cure sleep ASAP, it will waste a turn using Dream Eater. Also, it knows Thunder Punch and Fire Punch--the latter can annoy Bug and Steel-types. Dark-types clearly have the advantage here.
Jynx is halfway between Slowbro and Hypno in terms of levels of annoyance. It can use Blizzard and Psychic for STAB, Shadow Ball to damage other Psychics, and Lovely Kiss to cause sleep. Besides Dark-types, Fire-types would also be useful here. Steel-types can wall it fairly well too.
Finally, Alakazam is Sabrina's finest Pokémon. This one can use Psychic for STAB, Shadow Ball to counter opposing Psychics, Dazzling Gleam to punish Dark-types, and Tri Attack. It's fast and hits hard, so be careful. But it's quite fragile, so if you can sneak in a few hits, you should be good. Sucker Punch is great here since it can only attack.
Gym Leader Blaine
If Sabrina caused you headaches, Blaine is a far more simpler beast. Like before, he focuses on Fire-type Pokémon--and they're all pure Fire-types, so preparing a counter-strategy is very easy. Just bring Water and Rock-types.
To begin with, Blaine sends out Magmar. It can use Fire Punch for STAB, Thunder Punch to damage Water-types, Brick Break to get rid of protective barriers, and Will-O-Wisp to burn. Despite Thunder Punch, Water-types are ideal here. Rock-types are technically safer, but they'll be crippled by burn.
Ninetales can be your next opponent. This mystical fox can use Fire Blast for STAB, Dark Pulse to potentially cause flinch, Quick Attack to try and attack first, and Solar Beam to charge up an attack that hurts Water and Rock-types. If they're fast or strong enough, a good Water or Rock-type can easily deal with it.
Arcanine is Blaine's prized Pokémon. This one can use Flare Blitz for STAB, Play Rough to hurt Dark and Fighting-types, Crunch to take a bite from Ghost and Psychic-types, and Outrage for other targets. It's got nothing on Water or Rock-types though.
Rapidash is deceptively quick, so watch out for that. It can use Flare Blitz too, plus Quick Attack to go first, Megahorn to hurt Psychics, and Drill Run to wreck Rock-types. That leaves Water-types as the safest choice. Or you could go with a Ground-type.
Finally, Blaine has a Flareon--a Fire-type Eevee. This one can use Flare Blitz for STAB, Superpower and Iron Tail to knock down Rock-types, and Quick Attack to finish of weakened foes. Obviously, Water-types will have an easy ride. Like Rapidash, you could consider a Ground-type as well.
That's the first seven Gym Leaders, but what about the eighth one? Well, Giovanni has retired and you won't be seeing him again anytime soon. But if you go back to his Viridian City Gym, you'll find that somebody else has taken his place. If so, refer to our Battling Red, Green and Blue page.