To earn yourself a cool 3000 credits all you need to do is complete 50 rounds of Firefight in matchmaking.
Remember that each Firefight match in matchmaking is potentially 5 rounds, so completing this only requires 10 separate matches. After testing it looks like each MATCH is one ROUND. So that's potentially a LOT of Firefight. As always if you don't feel like taking your chances with the general Halo public you can give the Co-Optimus forums a try to find your Halo soulmate.