Doryoku Shisugita Sekai Saikyou no Butouka wa, Mahou Sekai wo Yoyuu de Ikinuku.

Doryoku Shisugita Sekai Saikyou no Butouka wa, Mahou Sekai wo Yoyuu de Ikinuku. (努力しすぎた世界最強の武闘家は、魔法世界を余裕で生き抜く., The World's Strongest Fighter Who Tried Too Hard Living Leisurely in a World of Magic) is a Japanese light novel series written by Wankosoba with illustrated by Ninomotonino and is based on the light novels of the same name. The light novel series was adapted into a manga series with illustrations by Nagatsuki, Masami and serialized by Suiyoubi wa Mattari Dash X Comic, with 6+ volumes released as of July 11, 2018.

Synopsis
One day, a martial artist named Ash was suddenly reborn into another world. He decided that he will become a sorcerer in his second life. He went through harsh training after becoming the apprentice of the former hero, Morris. Then, the "Emperor of Darkness" suddenly appeared! Right when the end of the world was approaching, he one-shotted the demon lord?!

(Source: Shueisha, translated)