this works just tested it on easy

"On the last mission in co op you can get an inf score. Once you leave the train tunnel and go up to the street with the cops, don't go inside. There will 2 enemies on each side of the entrance who will always respawn when killed. Me and my friend hide behind the cop cars for 30 minutes and got 80k+ score.
WARNING: Do not touch the step or else the guys stop spawning."
do we get more points on higher difficulty ?