So, who is a good example of a high level fighter who does extraordinary things and comes from mundane origins (ie no demigods or aliens or the like)? Who is the level 15 fighter emulating?
I ask because most of the best high level fighters (Batman, Captain America, Hercules, etc) get their abilities with a mixture of tech, external power (supersoldier serum) or origin. The purely mundane fighters are basically low level like Boromir and McClain. The closest I can think of is MCU Hawkeye and he admits he's hopelessly outclassed in the Avengers in terms of power.
My brain just doesn't accept the notion that Boromir eventually becomes Hercules if he manages to kill enough Orcs to get past 4th level and beyond. I can't make the jump from "steward of Gondor" to "demigod capable of rerouting rivers" without something setting them apart from normal people.