None of the mainline Pokemon titles are particularly difficult. That's why players often look for secret side quests in them, or when that proves too small a challenge, just make their own.
There are many challenges associated with Pokemon games that make them more difficult. Perhaps the most well known and widespread is the Nuzlocke challenge, where any Pokemon that faints is gone for good. Twitch plays Pokemon proved to be a popular one as well, with a Twitch chat controlling the player's every move. But one YouTuber decided to take the idea of other people controlling a Pokemon trainer to the next level by substituting a group of people with a group of fish.
Yes, in a record breaking moment that few people even imagined occurring, a fish has beaten Pokemon Sapphire. Or, more accurately, a team of fishes. Four fish, switched out every 12 hours to take a rest and recuperate, played the game through controls hooked up to their tank. As the fish swam around, they triggered the buttons on a Game Boy Advance. It took a long time (3,180 hours), but the fish team eventually steamrolled the Elite Four and Champion Steven.
To be clear, this is not a player-based challenge like beating Dark Souls 3 with a Guitar Hero controller, or even like Twitch plays Pokemon. This was based on completely random chance and patience, more of an experiment than an actual gaming experience. It definitely speaks to the technical prowess of the folks over at Mutekimaru Channel, as well as their fish handling. It also speaks to just how easy Pokemon Sapphire is if it could be defeated by a team of fish.
The fish team's secret weapon to defeat Steven was, perhaps appropriately, a Wailord. Obviously not a shiny, Dynamaxed Wailord, but for how effective it was it may as well have been super-powered. A bevy of ice attacks plowed through Steven's team, leaving him crushed beneath the giant Pokemon's might.
The layout shown for how the fish control the game looks simple enough, and it's now impossible to deny a game can be beaten this way. This raises the question of what else fish can do in video games. Maybe next the Mutekimaru Channel will have their fish catch fish in Animal Crossing: New Horizons of Fortnite.