Use Mission 7 of Spec Ops (Operation 3) to boost it up. Play fast in order to get the Multiplier x4 to get points faster, try to kill all enemies, and get all dog tags for extra points.
Instead of accumulating all the points u need for the trophy, buy the upgrades little by little so soldiers get stronger to get u more points.
In one playthrough of the mission, u can get an average of 150-200k points.
I preferred not to use the C help so I wouldn't lose points, but that was a stingy choice of mine 😉.