Oxygen.

I second the recommendation, it's a great story. The writing isn't perfect, but it's absorbing and there's plenty of good world building and characters.

There's another series like that, the Human Chronicles Saga by T.R. Harris. Humans are super strong because the gravity on Earth is much higher than on other planets. Materials on the other planets are weaker too, simply because they don't need to be as strong, so the humans can break through walls easily.

We are a species that came to prominence by literally throwing ourselves at monsters until we won. Injuries (such as a giant beast's foot crashing at you at 80KmH) that would kill any other species 100 times over is basically a little medical treatment and a month of rest away.

Fun (and also not fun) Fact is that the reason why bullet caliber exists is because when settlers were doing their genocide they would often unload their entire magazine into warriors to no avail, they would just rush up unimpeded and chop the settlers to ribbons. The concept of stopping power was literally invented because of the sheer resilience of humans and the amount of damage we can take and survive.

The only lifeform on this plant which can take anywhere near the level of punishment we can are insects, and in those cases even one injury is often enough to permanently disfigure and condemn them to a crapshoot of maybe being able to reproduce before they eventually die, because for as resilient as they are, nature didn't really build them for long distance survival like we did through our monster-hunting exploits.