Invincible: Slaying Demons in Tokyo

This book is also titled "I Spent Thirty Years on a Deserted Island Cutting Down Swallows, and Became Invincible the Moment I Left."

After transmigrating to ancient Japan, Hanyu Tsukasa received a beginner quest from the System. He was supposed to use a sword to cut down a swallow in flight.

Easy, Hanyu Tsukasa thought.

Until he realized he didn't have a sword.

And there weren't any swallows in the sky.

Anyway, it's a story about a boy cutting his way through everything with a sword in a world that's a complete hodgepodge.