Ash uses Infernape to defeat Paul’s Electivire in Pokémon the Series: Diamond and Pearl.
The battle takes place during the Lily of the Valley Conference, the Sinnoh Pokémon League tournament. Infernape’s victory is especially significant because Paul originally owned the Pokémon when it was still a Chimchar and later released it after judging it too weak.
Ash catches Chimchar and helps it develop into Monferno and then Infernape. The Pokémon’s Blaze ability, emotional resilience, and bond with Ash become central themes of the Sinnoh storyline. Its final confrontation with Paul’s Electivire resolves the trainers’ long-running rivalry.
The answer is not Pikachu, even though Pikachu is Ash’s most famous partner. Pikachu fights many important battles, but Infernape is the Pokémon that delivers the decisive defeat of Paul’s Electivire.