Start in DxD as Motohama: Leveling Up in Another World - Chapter 21: New Class

Step into a world where science meets the unknown, where mechas clash in brutal duels, and where every Biocore holds a mystery waiting to be unraveled.Follow Nioh, a genius weapons designer, as he uncovers lost technology, battles biomechanical titans, and pushes the limits of power, in an intergalactic thrilling adventure

There was once an immortal cultivator fighting for the fate of human beings. Unfortunately, he failed that time; but fortunately got another chance as he was reborn once more as a human, on the earth of 5,000 years ago. With a second chance, he not only would embark on the road of saving all mankind, but also would retrieve his human nature on the new journey that would last for 5,000 years.

- Description from Novelupdates

Ryu Han-bin, the main character who fell into a new world after being chosen as a ‘suitable person’.

Because of the error in the guidelines, the training is repeated endlessly…

The worst student of Westroad Academy. The disgrace of the Blake House. A new soul enters the body of Frey Blake, who couldn’t overcome his miserable life and chose death instead. “What’s with this body? I’ll have to work on tuning it from scratch.” The Great Mage Lucas Traumen, with the body of Frey Blake, gains a chance at revenge!