Although the achievement's description says you must earn over 9,001 points, it fails to say that the points are the same thing as Experience Points. So, technically, you must earn 9,001 experience points total.
Whenever you are in a Free Roam session, you will earn experience points by doing various activities. Completing Hideouts, killing civilians or killing other players will earn you experience. Whatever method you choose, it will be a bit of a grind to earn over 9,001 points. You can check your progress by hitting up on the D-PAD.
Completing Pike's Basin (hideout) is the fastest way to earn points. By completing the hideout, you earn anywhere from 600-1500 points. How much points you earn depends on how well you complete the hideout. Killing more enemies, not dying, etc. All of these effect your points.