In a World Dominated by Levels

One day, the high school boy, Tohru, was summoned by an aristocrat to [a world where levels rule]. Upon discovering that Tohru's level was the absolute lowest, he was then treated as a slave and trapped in a high-level dungeon. He vowed to defeat the fearsome monsters in the high-level dungeon, level up higher than everyone else, and return to his original world. Source: MU