To battle the strongest Pokemon, first you have to completely finish all of the cups in Stadium Mode along with Gym Leader Castle. Night falls on the stadium, and Mewtwo floats above, waiting. Move the cursor over Mewtwo and press A to enter the battle. You may use up to six Pokemon. The Doduo upgrade doubles the speed of the play of Pokemon Red, Blue, and Yellow on the N64.
Speed is now even faster.
By pressing C-Right during the game in Pokemon Red, Blue, or Yellow, you can switch between Doduo and Dodrio Modes. There is a secret R-2 Mode in Pokemon Stadium. After you have beaten the Stadium and Gym Leader Castle Modes, beat Mewtwo. After the credits roll, the R-2 Mode will be unlocked. It is much harder and has new battles.