Shikkaku Mon no Saikyou Kenja

A powerful sorcerer known as the Sage seeks the strongest way to fight, but discovers that his body is unsuited to magical combat. Rather than give up, he uses magic to prepare his soul for rebirth in the distant future. In his new life, he finds a world transformed—and magical knowledge has fallen to an extraordinarily low level. The Sage must now face this changed era with the experience and abilities he has carried into his rebirth.