In "A Fated Face-Off!" Misty and Ash compete to catch a Clauncher on a beach. Ash used a Cilan-themed bait and Misty used her secret Pokémon lure to begin their hunt.
Unfortunately, Clauncher pulls them into the sea instead of catching them. After Team Rocket interrupts, Ash and Misty resolve to fight for Clauncher.
Misty chooses Politoed after Ash chose Corphish, surprising Pikachu, who expected a type advantage. Corphish drops a Crabhammer after battle, releasing Misty's Politoed Mega Punch.
Misty wins and acquires Clauncher, her last Pokémon. When Clauncher emerges from its Poké Ball, Ash is courteous about losing and trains with it and Corphish, much to Misty's dismay.
Pikachu could have defeated Politoed, but Ash chose Corphish because they were close. Pokémon enthusiastically followed Misty since she won fairly.
After this episode, Ash only fights Pokémon Hunters and Team Rocket, therefore it's a sad way to conclude his battling career.
Even the Monarch of the World Coronation Series can lose, especially if the battle leads to bad tactical decisions. Misty should know that Pokémon's Ash Ketchum is still Ash, regardless of championship.
Also See
Solo leveling is great and could be the next Titan attack.