There is an area near the beginning of the first area where you can farm points without having to worry about enemies hurting you.
Upon starting the level head through the door (
to open) and shoot the first enemy you come across (
again) now you come to a second door,
this is the area where you will be farming from, open the door using
and the door should hit the enemy standing by it (Be quick he wanders off) this earns you 1000 points, walk away from the door to your left until the door closes again and rush back and repeat the process (
Make sure you wait for the enemy to have gotten up again).
Do this until you reach 40,000 points and the achievement will pop.
Here's a video demonstration.