How To Unlock: Reach level 50.
You will need a total of 5,000,000 PP to reach level 50. This can be done by:
- Rescuing survivors
- Defeating psychopaths
- Killing zombies with Combo Weapons.
Since you will be playing through the game more than once to earn all the acheivements/trophies, you may not have to worry about this one directly. It is easy to get with some fun playthroughs in free-roam.