Youheidan no Itoshigo: Shinikake Koji wa Saikyou Shishou-tachi ni Sodaterareru

Young Will is an orphan with no one to rely on. Left dying in a corner of the slums, his life is saved by chance when a passing mercenary finds him. To survive, Will asks to join the mercenary band. He's given an entrance test designed to be impossible to pass—but his talent is recognized, and he's accepted as a provisional member of the kingdom's strongest mercenary corps!? Receiving training from the band members—who are among the world's most powerful fighters—Will's extraordinary potential begins to bloom. Before he knows it, even the veteran mercenaries come to respect him—!? The ultimate growth fantasy of an unprecedented disciple raised by invincible masters!

(Source: Kadokawa, translated)