In order to get this achievement, you must beat every "optional" trainer in the game (the ones with the crossed swords above their heads). As far as I can tell, the story fights and boss battles don't count, so once you have access to the Khan Woods, you can reach every trainer you need for this achievement.
There is currently no way to tell which trainers you are missing, though only new trainers (and the thieves in the Frozen Tundra) will challenge you if you walk in front of them.
I used this excellent video guide by PrimeraEspada91 on YouTube that shows the location of every trainer in the game:
One thing to note: at 11:50 in the video, he goes to an area on the Drake Isles that doesn't have a trainer in it, but there is actually one standing in front of one of the buildings at the top of the map. Apparently this trainer was added in a later patch, and you do need to defeat him to get the achievement.