Firefight modes are the best, especially with challanges, but make sure you play game with explosives (arcadefight), that way you can get more kills. plus, try to go out and get kills, dying really doesn't matter on most firefight games, so get out there and just kill.
Secondly, go to xbox.com/ and log in (how you would for transfering live to another xbox) then go get on halo waypoint (thru the computer) and find the custom challanges area, then set the challenges to firefight stuff. Then go onto your xbox and acess halo waypoint, then find your custom challanges and accept them, you will then get even more credits from those challanges as well as the game and normal challanges.
Lather, rinse, repeat.
BTW go make an intro thread, we want to know you better!