You can't. Also rare candies level up your Pokemon but they do not make them as strong as if they were to evolve by training them, yes it takes longer, but its better in the long run.
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Its simple you cant you have to have game shark.
gamedolphin! (this is a old answer) The new is, you could try codebreaker.
make a unlimited money on hooves of fire
how to get 99 rare candy
The item duplication glitch (or Rare Candy glitch) can't be done in Pokemon FireRed version.