Before battling Erika, visit Celadon Mansion to receive Eevee.
Eevee can evolve into three types: Fire, Water, and Electric, making it versatile against your starter's weaknesses.
If you chose Squirtle, evolve Eevee into Vaporeon.
For Bulbasaur, evolve Eevee into Jolteon.
For Charmander, evolve Eevee into Flareon.
Head southwest to Celadon Gym with a Pokémon that has learned "Cut."
Challenge Erika in the gym.
Erika's Pokémon "Red, Blue, Green"
Erika's Pokémon "Pikachu Version"
If you chose Squirtle, Eevee can be obtained on the rooftop in Celadon City.
Before challenging the gym, get TM 13 "Ice Beam" from a lady on the rooftop.
You can receive "Ice Beam" by giving the lady a "Fresh Water."
Celadon City 2
After defeating Erika in Celadon Gym, return to La vender Town and head to Pokémon Tower.
La vender Town
Battle your rival on the second floor of Pokémon Tower.
Be cautious, as there are many trainers in this area, and you need to manage your Pokémon's HP.
Battle with Cubone near the top floor's stairs.
To battle with Cubone, you need the Silph Scope.
After winning, defeat Team Rocket on the top floor and sa ve Mr. Fuji.
La vender Town 2
Receive the "Poké Flute" from Mr. Fuji's house.
Use the "Poké Flute" on Snorlax at Route 12 and Route 16.
Snorlax has high HP and Attack but relatively low Defense.
In FRLG, Snorlax has high Special Defense, making it challenging for Charizard.
Since Snorlax has low Speed, try to induce "Paralysis" and gradually decrease its HP before attempting to catch it.
Challenge the Fuchsia Gym.
Visit the Friendly Shop and purchase items such as "Antidote" and "Revive" in preparation.
Battle the Gym Leader Koga in the back.
She uses Poison-type Pokémon and the move "Toxic," which increases damage each turn, so be cautious.
Koga's Pokémon "Red, Blue, Green"
Koga's Pokémon "Pikachu Version"
Koga specializes in Poison-type Pokémon.
Poison types are weak against Ground types. Using Pokémon like Cubone, Nidoking, or the recently awakened and caught Snorlax will make the battle easier.
For Snorlax, use "Amnesia" to increase Special Defense, a void "Toxic" with "Rest" while asleep, and attack when the opportunity arises.
After defeating Sabrina in the Saffron Gym, go to the Safari Zone and find the Treasure House in the back.
Obtain "HM03 - Surf" inside the Treasure House.
While you're there, pick up the "Gold Teeth" near the Treasure House.
Return the "Gold Teeth" to the Warden's house to receive "HM04 - Str ength."
Fuchsia City 2
In Fuchsia City, obtain the "Good Rod" from a fisherman.
Here, you can fish for higher-level Pokémon than with the "Old Rod." If you want to create a Gyarados, be sure to get it.
Additionally, visit the observation deck on Route 15, where Oak's Aide will give you the "Exp. All."
You need to ha ve caught 50 or more Pokémon for Oak's Aide to give you the "Exp. All."
Evolve Pokémon or catch various species in the Safari Zone to meet this requirement.