GLITCHED - it is highly recommened to not play co-op until you've gotten this and the below trophy (or create a back-up save to return to if you do). Co-op is extremely broken and tends to permanently freeze your challenge progress. You'll either need to reload a past save before the challenges bugged, or start a completely new save.

To unlock perks you have to complete challenges. At first you'll unlock a perk after every challenge completion but eventually you'll need to complete two challenges to unlock the next perk. To unlock every perk you need to complete a total of 46 challenges.

There are well over 70 challenges, so you have a good amount of choice in what to complete, you should unlock a decent amount on the way to the other trophies and make good progress on some of the others depending on your play style, such as whether you switch out weapons and such as you progress through the game. There's a lot of challenges that just require you to get a bunch of kills in a certain way or with certain weapons, and you can make enemies come after you infinitely from the Radio Venture so you can easily farm them.

Once you've completed 46 challenges and unlocked the final perk this will pop.