Annoyed at having 3 fistfights in a row without being able to save? Too low-level for the fight, but it's part of a quest-related section and you can't attempt it again? Try the Fistfight Cheat mod.

-=Description=-
This mod makes Geralt invulnerable during nonlethal fistfights. This means you won't take damage from opponents, and can finish them at your leisure. After the fistfight is over, Geralt is restored to normal vulnerability. It doesn't influence fistfighting outside the minigame.
Also, it won't help you in the fight with Olaf or the Champion of Champions, since those seem to be coded differently - not your regular fistfights, after all.

-=Installation=-
Extract into ./mods/ .
If you don't have the mods directory, create it as <Witcher3 install location>/mods , then extract this mod there. The content subdirectory should be:

    <someplace>/The Witcher 3 Wild Hunt/mods/modFistFightCheat/content

-=Conflicts=-
It modifies scripts/game/npc/npc.ws , taken from game. To have it run alongside other mods modifying this file, run a script merger.

-=Implementation=-
This mod adds two lines of code to scripts/game/npc/npc.ws. In function event OnStartFistfightMinigame(), it adds, as the second line:
    thePlayer.SetImmortalityMode(AIM_Invulnerable, AIC_Fistfight);
to turn on invulnerability.
in function     event OnEndFistfightMinigame(), it adds, as the first line:
    thePlayer.SetImmortalityMode(AIM_None, AIC_Fistfight);

This method should work for virtually all game versions, if you make the changes manually.