I was constantly stuck on about 38,000 points upon completing this level. Then I discovered the best way to reach 40,000 isn't to be quicker, but to be more violent.
I would go through the level normally, grabbing all the loot. Instead of going up to the roof, though, go down to the first floor. When there's a room with 2 normal cops, just keep shooting them. You get some points for each time you KO them, and it should be enough to get you over the hump.
FYI: I don't even think you need to beat the level to get the achievement.