Toaru Majutsu no Index - Volume 12, Chapter 4: Two Pairs Gently Cross Paths. Boy_Meets_Girl(X2).

In the early world of immortal cultivation, there was no physical cultivation among immortal cultivators. And a top bodybuilder who came from the earth has opened up a new path here and became the ancestor of physical training.

- Description from novelbuddy

As a successful villainous BOSS, in addition to having ideas and aspirations, one must also keep in mind three points:First, grow sneakily, don't act rashly (once a villain acts rashly, they basically won't survive past three chapters).Second, you're not a good person, so be ruthless (becoming more evil will make you three times stronger, while redemption weakens you by seven times).Third, remember the two points above. The moment you forget them, your end is probably on its way.

Mira is determined to be a botanist—no matter how dangerous or violent it may become. Mira made two requests upon entering the multiverse: a botany class to further her research and to be dropped off in an isolated forest far from the violent god-rearing system BS going on. She was granted both—but not in the way she expected. The system abandoned her in a hellishly dangerous forest—where she earned a rare class that excelled at killing things.