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

The only reason he is the strongest is... effort!! Ichitaro, a high school boy, has a personality that causes him to get completely absorbed in a single thing. 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, Ichitaro is locked away in a prison where time has stopped. What he did there... was training for 100 million years. By the time he left the prison, he had mastered each and every swordsmanship style, becoming the strongest hero!!