Ok there are 5 types of difficults in the game: Very easy, Easy, Medium, Hard, Insane. The dificulty "insane" earns you the most credits (duh!). Anyway you should go to Quick match and set it like this:
Map: Dome (night) Blue team players: 6 Dificulty: Easy Orange team players: 3 Dificulty: Insane Song: Rose of Nightfall type: Deathmatch kills to win: 15\\20
Playing "insane" will let you have credits-fast! Or try this
Map: Mines blue team players: 4 Dificulty: Easy orange team players: 6 Dificulty: Easy kills to win: 100 song: Rocket Race
