PP is gained from all modes in the game just by playing, win or lose. The better you are, the more PP you will gain.
You can gain between 2,000-3,000PP by playing through Arcade Mode on Very Easy.
Winning a Ranked Match online will get you about 300PP.
Completing one mission in Mission Mode will net you roughly 10PP.
A couple of minutes of Training will get you about 100PP.
If you complete the game with everyone for the achievement "Waiting for the Trade", it should be enough to get you 100,000PP.