I Kept Pressing the 100-Million-Year Button and Came Out on Top, Vol. 1 (manga)

Allen gets such poor grades at Grand Swordcraft Academy that his fellowstudents have
dubbed him the “Reject Swordsman.” But one day, heis granted a mysterious button
that, when pressed, will give him one hundredmillion years to train in an alternate
reality. With an ungodly amount ofpractice under his belt, the world is about to see
what this underachievercan really do!