The Demon King's Champion

Once a hardcore gamer, 75 is now a full-fledged hero inside his favorite game! After one too many all-nighters, he dies of a heart attack at his computer but finds himself reborn in the middle of a boss fight. Unequipped and lacking the skills to survive on his own, 75 uses his smarts to convince Devo the Demon King to take him under his wing. Now 75 is on an epic quest to help the Demon King stay in power! To get there, 75 has to level up and conquer all his enemies, but if he doesn’t, this might just be his last life...

(Source: Tapas)