All achievements are done and or explained in this video.
For this achievement we are going to load our save of our completed game and head back to the Sierra Morena via fast travel. This will take you to the exact location of where you need to be. Select Uay Chivo as a costume and find the defeated Jaguar still writhing in pain. Hit him to challenge him to a rematch
Uay has infinite stamina so spam derp derps into him and he'll be gone in no time.
The video starts with this achievement
Piniski47 reminded me "You have to do this single player. If you are playing multiplayer, all the other players need to drop out."