After a meteor (Or something) hits a hill outside Ravensthorpe, you can follow a side quest which will include a fight, then the game will reveal the Mastery Challenges, then simply complete one with a gold medal score.
To get a gold medal you need to get 1000 points, which is only possible by strictly following the scoring outlined in the pause menu. So for example if you need say 330 points for poison kills or what have you, once you've hit that score you can then move onto the next requirement. Once you've hit the required score the objective will be removed from the upper left corner of your screen, so it should be fairly obvious.