Saiteihen no Ossan Boukensha. Guild wo Tsuihou sareru Tokoro de Ima made no Doryoku ga Mukuware, Kyuu ni Saikyou Skill "Mujouken Shouri" wo Eru

Adventurer Lewis is nearing 40 and still stuck at E-Rank. Mocked by his guild as the "Immovable E," he’s become something of a laughingstock. One day, while fighting with his life on the line, Lewis manages to defeat a monster far above his level. As a result, his level increases for the first time in his life, and he awakens a new skill—the legendary "Unconditional Victory," once wielded by a hero of old. What exactly is the true nature of this skill that can mow down massive enemies with a single blow...!?

(Source: Futabasha, translated)