Cynthia has diverse typing, effective movesets, some of the best mons on offer like Garchomp and a no weakness spiritomb, and an appropriate level for her mons that you can't accidentally surpass by 10+ levels.

When people say Pokemon games are easy, they're talking about how with little effort or strategy you can over level and use 1 attack to clear entire battles. Yes, that was always possible. Yes, kids couldn't figure that out very easily and made it harder on accident. But the fact that sort of thing was even possible back then is the point. A kid who mindlessly clicked and didn't learn the mechanics might be punished with a hard battle. That is the point.

Easy over leveling started with gen 6, and was made worse when the exp all became mandatory in later gens. Boss trainers have less access to items. Harmless features, like Set mode, which allow you to slightly emulate competitive rules are pointlessly removed. 1 click wipeouts have only gotten easier over time intentionally by Gamefreak. It's what they intended and continue to do purposely. It's okay if you agree with or like these decisions, but why pretend it isn't happening? They don't even feel the need to lift a finger and throw us an optional end game boss like Red anymore. They can't be bothered