Tsuihou Sareta Fuyojutsushi, Saikyou no Jouzan Fuyo de Nariagaru: Boku no Fuyojutsu wa +20 janaku x20 nan desu

This is the tale of a young man who dreams of conquering a S-rank dungeon to fulfill a promise he made with his childhood friend with the help of his Multiplication Enhancement Magic. Desmond, a D-rank adventurer who can only use basic enhancement magic, has a secret. Unlike normal enhancement spells that increase stats through addition, his ability is based on multiplication. Not +20, but x20! However, his party fails to understand his ability's true power and banishes him from the party. Just then, his childhood friend Cynthia, a S-Rank adventurer, appears before him. Accepting her offer, Desmond joins the top-tier clan Chevalier Garden. With his enhancement magic, he begins taking down fearsome dungeon bosses, clearing dungeons at impossible speeds, and achieving success.