Question

Is it possible to beat the game with just one Pokemon?

Answer 1

Yes it is, but it can be very difficult. Most speedruns are done with one Pokémon (Nidoking is the most popular for Gen 1 speedruns), and you can find a plethora of solo runs with just a bit of searching. Solo runs use a lot of X Items and generally involve a LOT of tries, especially with harder Pokémon.

Cosmog solo run.
Ditto solo run.
Spinarak solo run.
Shedinja solo run.

Comments

By X items, do you mean items like charizardite x?

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He means items like X Speed, which raises your speed by one stage