Spirit-Taming Boy: From the Start, I Fuse with the School Belle to Slay the Demon King

Demonic caverns have invaded, corrupting everything. Humanity has entered an age of cold weapons, and a tiny fraction of people have awakened special abilities, commanding all kinds of demonic beasts in battle to protect the survivors.

Ye Feng transmigrates back to ten minutes before the demonic invasion and gains a special spirit-commanding ability: anyone he beats grows stronger. He awakens the campus belle as a giant, and his spirit-commanding power makes her combat strength soar again, letting her slaughter the Demon Kings as soon as they invade. His deskmate turns her left arm into a giant shield and her right into a Gatling gun, then upgrades into a flying shield and a close-in weapon system after he commands her. Another classmate turns into an off-road vehicle, then evolves into a super tank with his power boosting her. His little sidekick turns both hands into battle blades... As they kill monsters and absorb demonic energy, they upgrade into planet-destroying cannons, battleships, super fighter jets... Ye Feng, meanwhile, receives the benefits of their growth, gathering all that power within himself until his body reaches sainthood...

The survivors in the darkness see the light once more. When the Demon Realm fully opens and the demon clans invade, they discover that only a handful of people stand in their way, and burst out laughing... Ye Feng laughs too...

(The blurb doesn't do it justice. The story is even better!)