this also is pretty much self explanitory.. you just gotta keep playing and getting those 5 star kills in.. use traps as often as you can as they give you 5 star kills aswell, although i dont think that works if you use the same trap two times in a row, i did and the second time i used the same trap i only got 1 star..
you could also set the scene time limit to the max and ai to easy, that way you would rack up a fair few stars on scenes like statue, infected and hunt the leader.
update: this can be done in player/ranked/solo matches
update no2: ive managed to get in a few ranked matches now and to do this quickly i suggest boosting in ranked deathmatch with 1 other player. the player going for the stars should just find a trap near the center of the map and then have player no2 keep running into it.
i done this on the haunted house map with a friend and used the trapped room in center(room that seals shut and then gets fire blown out of the fireplace). my friend kept spawning and running in there and i would trap him, then when room opened back up i would run up to his corpse and humiliate him giving me 6 stars per kill...
when the player getting killed runs into the room, he needs to stand near the couch out of view from the guard by one of the doors, just so he doesn't bust you when your humiliating your dead friend, although that still might happen a few times as there is a guard on the floor above doing his rounds that might spot you.. no big deal really as his stun wont kill you and you only lose 1 star. at the end of one 20min match i had racked up 259 stars, with room for improvement..
so if you take turns at maches and move fast in the matches i think you could get near the 300 star mark per match, you and your friend should both have this cheevo between 4 - 5hrs and thats with both players starting with 0 stars each... if you've already got some stars then obviously this will take less time.