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Playing with your starter in any game is playing on easy. Starters as a whole have way better stats than the majority of Pokemon, and the later the generation, the bigger the power gap between the starter and whatever Pokemon you're facing. If you actually want a challenge you'd box your starter ASAP and go with the regional rodent or bug.
The only exception to this is Chikorita due to it having losing matchups against most Johto gyms and its playstyle leaning towards support, requiring proper team building to back it up, but this is the only example.
Also SWSH is impossible to be challenging because of forced EXP share.
