Once the Gym Leader Castle and every one of the Cups in the Pokémon Stadium have been cleared, you will unlock one more challenge.
You will be able to face Mewtwo, who was bar none the absolute best Pokémon in RBY. There was no other Pokémon with stats as high as Mewtwo's, and he had few "counters"; many of his counters required support from one or more Pokémon. However, Mewtwo has a diverse move pool, and it can beat many, or even all, of its counters with the right move set.
In short, Mewtwo was an absolute terror in RBY. Thankfully, its power was reduced in later Generations, starting in GSC, where the Special split significantly lowered Mewtwo's Special Defense, and Steel and Dark-types were introduced to counter the overpowered Psychics in the game.
Back to the battle, you will face Mewtwo and only Mewtwo; you can bring between one and six Pokémon to face it. His R-1 battle isn't too difficult, but his R-2 battle is definitely a challenge.
Electrode was there to just paralyze Mewtwo and the use Explosion. I completely forgot that Mewtwo had Rest instead of Recover. Zapdos definitely could have taken on Mewtwo, but that freeze prevented that from happening.
Slowbro also could have taken out Mewtwo, but in his last few turns, it couldn't move. However, Slowbro damaged Mewtwo significantly, enough for Dragonite to finish it off. The way Mewtwo faints is awesome, as you'll notice.
I used:
Electrode (nuclear), Zapdos (Crackle), Slowbro (Speedy), Dragonite (Drago), Gengar (Shadow, unseen), and Charizard (blazer, unseen).
With the Gym Leader Castle and Mewtwo battle in Pokémon Stadium cleared, I'll move on to Pokémon Stadium 2. However, that won't be happening for some time. But stay tuned, and thanks for watching!
