Shikkaku Mon no Saikyou Kenja

In a certain world, there was someone who excelled in magic combat, and people called him "Sage". Seeking the strongest combat art, he devoted himself to research every magic and combat art in existence, and the conclusion he arrived at was very cruel: "My body is not fit for magic combat." Yet he didn’t give up. He sealed his own soul with a magic art and was reborn in a far-off future, and the magic theory in the completely changed world he saw there was of unbelievably low level.