One Piece: Trainers on the Straw Hat Pirates.

He traveled to the world of One Piece and obtained a cheat code that allowed him to enter the Pokémon games.
I thought I could summon an electric rat, a garlic turtle, and a fire-breathing lizard to wreak havoc in the pirate world.
The protagonist, upon entering the game, was utterly astonished: "Luffy!? Why are you in my Pokémon list?!"
………………
Marineford War.
"Let our bond become our strength, and charge forward! Luffy!!! Mega Evolution!"
Nikka Luffy, reporting for duty!
"Give it your all! Focus completely! Cut down all obstacles! Zoro! Use Z-Move!"
Three-Sword Style Secret Technique Z: Ultimate Destruction Flying Dragon Slash!
"Franky Gigantamax! Franky General Arrives!"
The signal of fierce battle signals the arrival of the mecha era!
Nami wielding her Clima-Tact unleashing a 100,000-volt attack, Usopp sweeping the field with his Seed Gun, Sanji transforming into a fire knight and dancing among the crowd...
When the Straw Hat crew, having gained the power of Pokémon, arrived at Marineford, all the powerful warriors then discovered...
It turns out they had underestimated that man's power all along!
……
This book is also known as: "I Play Pokémon in One Piece" and "Because I'm too afraid of getting hurt, I made my partner the strongest".