Akademii no Kenjutsu Shinan ni Natta

When a failing web novelist drops dead at his desk, he's granted a wish in the underworld... and uses it to enter a novel. But instead of becoming the main character, he possesses Kang Shin-Hyuk, a total extra! Now set to be the protagonist's future teacher, Shin-Hyuk's out to use what he knows of the original plot to borrow the hero's power-ups for an easy, carefree life. But in a world of monsters and danger, can Shin-Hyuk really rest easy? Or will he be made to buckle down and hustle once more?

(Source: Tapas)