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The Ratical Only Challenge is a challenge in which you use only Raticate to beat the game. In Pokemon Crystal, Erika is the fourth gym leader in Kanto and the third gym leader in Johto. She specializes in Grass-type Pokemon. I couldn’t find any information on whether there is a Ratical Only Challenge against Erika specifically.
The Ratical Only Challenge is a challenge in which you use only Raticate to beat the game. In Pokemon Crystal, Erika is the fourth gym leader in Kanto and the third gym leader in Johto. She specializes in Grass-type Pokemon. I couldn’t find any information on whether there is a Ratical Only Challenge against Erika specifically.