First, I would like to note that the below strategies are for beating Blue, and only for beating Blue. These strategies will not necessarily help you against “real players” and are no intended to be used as a substitute for proper team building. That being said, rather than relying on specific Pokémon these strategies suggest Pokémon based on both type and statistic advantages so the player can choose the Pokémon and moves that best fit their playing style.
Most of Blue’s teams are similar and they almost always include some, or all, of the following Exeggutor, Arcanine, Gyarados, Rhydon, Alakazam, and Pidgeot. Therefore, my main strategy for defeating Blue is going to be based on defeating, some, or all, of these Pokémon.
The first Pokémon you should consider having in your team is an Electric Type. The electric Pokémon you choose should have a balance between physical and special attacks because using both allows you to take down both his Gyarados and his Pidgeot with ease. It is best to rely Special moves with Pidgeot however, either move type can be used to fight Gyarados.
Useful Electric Type Pokémon: Ampharos (Sp.Atk) and Electivire (Sp.Atk).
Others to Consider: Lanturn (HP), for its dual Water/Electric type advantage and Rotom(Sp.Atk/Speed), for its Ghost/Electric Advantage. Both of them provide a unique type combination that gives them an advantage.
The next Pokémon that would be advantageous to have is a Water Type Pokémon. It doesn’t matter if the water Pokémon use Physical or Special Attack moves specifically because the Pokémon that you will use them on should fall easily to any water attack. The Pokémon that can be defeated using water Pokémon are Rhydon and Arcanine.
Useful Water Type Pokémon: Blastoise (Sp.Def)
Others to Consider: Swampert (Atk), provides a Ground Type in addition to water which can be useful against Arcanine and Rhydon. Kingdra (Atk/Sp.Atk/Sp.Def) provides a Dragon Type Advantage which can provide a good defense. Once again, Lanturn would be useful as a Water/Electic type.
You should also consider using a Grass Type Pokémon which could easily take down Blue’s Gyarados and his Rhydon. Once again it doesn’t matter if your Grass Type Pokémon relies on Physical or Special Attacks. The attacks will be especially strong against Rhydon because both of his types are weak against Grass Type.
Useful Grass Type Pokémon: Meganium(Def/Sp.Def), Sceptile(Speed), and Tangrowth (Def)
Others to Consider: Venusaur(Sp.Atk/Sp.Def) or Vileplume(Sp.Atk), the poison won’t hurt your team and may even help against Exeggutor
Another good Pokémon Type Choice is either a Dark Type or Ghost Type, both will help against the Psychic Exeggutor and Alakazam however, a ghost Pokémon will also be able to protect itself from Pidgeot, however, it also isn’t particularly strong against Pidgeot either.
Useful Ghost or Dark Type Pokémon: Gengar(Sp.Atk), Umbreon(Sp.Def), Absol(Atk), or Houndoom(Sp.Atk)
To finish out your team I would suggest getting a Fire Type or a Dragon Type. Neither of them will be particularly helpful however, they are could be useful against one or two of Blue’s Pokémon. A fire type can be used to round out your team far better than a dragon type which only provides a defensive advantage, so using a Fire Type may be a better choice.