Ichiokunen Shugyou no Mujikaku Kensei: Tada no Bou da to Baka ni Sareta Bannou no Jingi to, Kyokugen no Shugyou de Umidashita "Kenki" no Jinkaku de Isekai wo Attou suru

The only reason he is the strongest is... effort!! Ichitarou, a high school boy, has a personality that causes him to get completely absorbed in a single task. One day, his entire class is transported to another world as heroes. The hero rank assigned to him at his destination is the worst. Deemed a useless existence, Ichitarou gets locked away in a prison where time is stopped. There, he devotes himself to 100 million years of training. By the time he leaves the prison, he has mastered each and every swordsmanship style and become the ultimate hero!!

(Source: Kodansha, translated)