Attaining Immortality, Starting From Slaying Demons https://www.webnovel.com/book/attaining-immortality-starting-from-slaying-demons_28888754100262105
protagonist is op but not invincible, and his cheat unique too. he can exchange lifespan/demon lifespan(he need to kill demon to get demon lifespan) to proficiency skill or cultivation. world setting is quite nice and emotional seeing humans fight against demons who are intelligent but consider humans as food.