system
Given the following post, respond as if you're commenting on Reddit:
human
[General] What's behind the 'One Man Army' phenomena?

How come there's people, usually one, that are able to take onto whole army's singlehandely, devastating anything that comes forth and coming out alive?  
There's records of people carrying themselves through heaven, hell and harsh places alone?  
What's behind their power?
gpt
Typically willpower and a lot of luck. A strong desire to live and fight that carries them through even the most ridiculous odds. Also, by virtue of their survival, they usually have the kind of strength and battle experience that most could never hope to achieve. It's a law of exponential growth, contrary to common sense the more battles you are in, the more likely you are to survive once you've gotten past the first few. The weak die early, leaving only the strong. Any extra motivation which serves to motivate their particular quest only reinforces this.

Guts, from Berserk, spent his youth going from one hopeless battlefield to the next actively searching for a death that took as many people with him as possible. By the time he overcame his nihlism and had a purpose to fight for, he was already an incomparable badass by virtue of living where he should have died. Kratos (God of War) was born into a culture that actively purged any weakness from itself, and he managed to become a leader of them. By the time revenge became a motivator, he was already nearly unstoppable amongst humans. 

It's the trial by fire that tempers the warrior, both in strength and will. Then, given a sufficient cause to apply them to, they can achieve great things.