Prestige Hound Achievement
Complete all PP Trials.
PP trials are challenge-type tasks that keep track of how many locations you've visited or enemies you've killed, depending on the trial. Some of them are unlocked right off the bat, while others unlock as you reach a certain point in the story or obtain the necessary weapon or vehicle. You will occasionally get progress pop-ups on screen as you complete the trials, and the trials themselves can be found under the "player" portion of the menus. There are generally far too many trials to complete in a single playthrough without extra effort, luckily they carry over between playthroughs, and Chapter select, allowing you to complete them at your leisure post-game along with your other achievements.
Check out my PP Trials list in posts 7 and 8 of my achievement guide (or scroll down if you're already here!) for tips on how to tackle the trickier ones.