It Seems That I've Slipped Into a Different World. Also, My Gender Has Changed. - Chapter 3 - Somehow, I’ll just wait in my room

How can there be any invincibility?

How can there be any Xeon undefeated?

No one in this world can be eternally number one, anyone can be defeated.

including me.

I’ve been waiting for that one person to show up, beat me, tell me that my way is wrong.

It’s a pity that that person hasn’t come yet.

…

False martial arts: strengthen yourself, break the limits of the human body, and resist bullets with flesh and blood.

Real martial arts: use the magnetic field of life to dominate the magnetic field of the stars, use the small universe of the human body to attract the power of the big universe, travel the starry sky, and smash the planet.

- Description from novelbuddy

When The Cute Kunoichis Spied on Their Teacher's Secret is a light novel written by Kazuma Kamachi and illustrated by Ryuushi Tajima.

The story takes place in a world where Japan never ended their isolationist phase, and agents from foreign powers are always threatening to steal the country's advanced technology. In order to combat this threat there has been a resurgence of ninjas that use Strategic Martial Arts, techniques that combine cutting-edge modern technology with a traditional foundation of martial arts and ninjutsu. The story follows Souha Sugiyado, a 17-year old ninja instructor and former ninja prodigy who was forced to retire from active duty by the wounds he suffered in his last mission. Souha's four students are all kunoichi who have all gained the rank of Elite Ninja, the highest official rank available to ninja. However, one day two Shogunate ninja approach him with the news that his four students have gone rogue and have stolen a portable modular nuclear reactor, and thus they ask for his help in bringing his students to justice.

“Ding! Soulbinding has been successfully completed; you will now head to the Heaven Awaken World.” After hearing this, Zhao Fu’s vision darkened as he fell to the ground.

With bleak prospects in the real world, Zhao Fu’s life is turned around when countless crystals fell from the sky one night, which people could use to enter an alternate, game-like world. After obtaining an ancient Chinese empire’s legacy, Zhao Fu uses his intellect and resourcefulness to develop his own empire from a tiny village. However, with enemies both in the real world and in the Heaven Awaken World, he must make brilliant decisions and use creative schemes to survive.

- Description from Novelupdates