To unlock the Fist of the South Star trophy in The Witcher 3: Wild Hunt you need to avoid all damage in a fair fist fight -- no magic or swords allowed.
The easiest and earliest opportunity for this is in White Orchard, the very first time you enter the inn. Finish your questioning inside and then head out (maybe save before exiting if you are not too confident in combat yet).
You'll be threatened by three guys outside. Tell them three against one isn't fair (for them) to draw one guy into a fight. Get ready to immediately dodge back with
before the cutscene ends as he throws a quick punch.
As long as you avoided the first punch, you now simply need to parry with
each time you see his name flash red and wail on him while he is staggered. As long as you don't take a hit the trophy will unlock.
If you didn't go for this trophy at the beginning of the game you can take your chances on several fist fights throughout the adventure, including a series of arranged fights in various locations that forms a long-running side quest. Otherwise you could start a new game on the easiest difficulty and play through the 30 minutes or so that it takes to get back to the above described fight.