Jackarandajam
Maester
MeatBricks... NightNight... Clock... Stars...If it were me naming a character, I'd start with simple but effective. I can see your guy here being called "Chaos," or "Trickster," Or maybe just "Dragon." Or we could go the Irish route and flip it around. "Honesty." "The Steady."
I don't like "Crack Shot" mostly because it's a very short jump to "Butt Crack" and you don't want to put that in your readers' brains unintentionally. Also, the term 'crack shot' is very closely tied to sharpshooting, so with guns, in American English and will sound strange on someone without a gun. For a character who's primary weapon is her fists I like "Hammer," or "Sledge," stuff like that.
The really good ones are based on stories though. Like with a boxer, maybe "Pinkie," cause during a fight a decade ago they actually got a whole pinkie finger ripped off, but still finished the fight.
Even if the pinkie finger got put back on and/or they lost the fight, that's the way nicknames work; somebody says it, somebody else repeats it, and boom, stuck with it forever.
And it would be super cool to have some gigantic cauliflower eared monster of a boxer named Pinkie.