Kougekiryoku Zero kara Hajimeru Kenseitan

Although Eugene was blessed with an extraordinary talent for defensive magic, he had no talent for offensive magic. He is ridiculed as incompetent by those around him and enrolls in a magic academy to escape. However, his "useless" defensive magic came in handy there, and as expected, he was assigned to take care of the beautiful Demon King Erinyes, who once ruled the world...!?

(Source: Overlap, translated)