Hell Mode: Yarikomizuki no Gamer wa Hai Settei no Isekai de Musou suru

Kenichi Yamada, a single, 35-year-old businessman, has spent the majority of his adult life playing every MMORPG to hit the market. Tired of the constant cycle of repetitive, newbie-centric gameplay, he is immediately drawn in by the promise of a new game that, on top of being never-ending, offers up a "Hell Mode"—a difficulty level that makes it nigh impossible to level up but also promises uncapped growth potential. Upon selecting the newly released Summoner class, Kenichi finds himself reborn as Allen, an infant in a serf family, with nothing but his wits and old memories to guide him. Allen works diligently to level himself up despite Hell Mode requiring vast amounts of experience between levels. In doing so, he hopes to uncover the secrets the Summoner class holds. All the while, he dedicates himself to freeing his new family from serfdom.

(Source: J-Novel Club)