This will unlock fairly shortly into the game if you do every challenge as it appears and you are going for the achievement for getting a gold rating on 50 challenges. A challenge is indicated by a green icon on your map. Most of the challenges can be gold rated fairly easily (therefore unlocking the bronze for you as well), and others are more difficult to get a gold rating on (but nearly all challenges can EASILY be bronze rated). If you just want this achievement, I recommend weapon challenges, traversal challenges, and "smash" challenges because all of these require minimal effort just to reach the bronze rating.
Traversal challenges will give you bronze as long as you have completed the challenge before time completely runs out.
"Smash" challenges require you to smash some of the items to reach bronze, but far from all of the times are required to get a bronze rating
Weapon challenges should be bronze rated as long as you are actively killing enemies
Like I said at the beginning, most of these challenges are actually very easy to get a gold rating on. If you are gold rating challenges, you will have this achievement long before the achievement for getting a gold rating on 50 challenges.