My Isekai Life: I Gained a Second Character Class and Became the Strongest Sage in the World! manga volume 15 features story by Shinkoshoto with art by Huuka Kazabana and Ponjea.

Yuji Sano was a normal guy working a soul-sucking nine-to-five job when he suddenly found himself transported to a fantasy world where he restarted his life as a Monster Tamer. After befriending the Slimes, Yuji is quickly able to gain a second character class, ranking up his stats and learning new skills as he encounters one surprise after another.

A seemingly unending horde of monsters threatens to take over Evildominus Island thanks to a mysterious cursed tree. Yuji must figure out a way to save the adventurers from a grisly fate without letting anyone find out about his powerful magic ability! While doing so, Yuji receives an unexpected letter from Father Steyl, giving him a convenient cover story for the extraordinarily powerful spell he'll need to wipe out the monsters and protect the precious fertilizer, Mother Earth's Tears. But will his magic be enough to dispel the curse that shrouds the island?