The Strongest Wizard Making Full Use of the Strategy Guide -No Taking Orders, I’ll Slay the Demon King My Own Way-

Magnus, a mage who always got yelled at "just shut up and follow my orders!" by the Hero because he was deemed "useless". One day after getting fed up with his party he left, upon leaving he met a merchant that sold him a mysterious book.
And it turned out that the book which was written in the god's language was actually an ultimate encyclopaedia book that had a lot of useful information on how to defeat the Demon Lord!
Thanks to that book, he was able to know about boss monsters' weaknesses and raise his level more efficiently, data about important people, and it even had info on complete gourmet spots in any city! Moreover, by using his wits and scheming mind, he's even able to outclass the hero party! With the ultimate encyclopaedia in his hands, advance to the true route to defeat the Demon Lord!

Magnus wasn't chosen by destiny to be the hero. That's why, He's going to beat the Demon Lord with his own power and wits and save the world.