Instructions
Upgrade your commendations. To view a list of commendations, select "Player" from the pause menu and then choose "Service Record." Campaign mode, Matchmaking mode, and Firefight mode all have their own lists of six-tiered commendations (iron, bronze, silver, gold, onyx, onyx max). Read the commendation descriptions and complete each task in the appropriate game mode to gain credits that can raise both your level and rank. In general, completing a commendation at the iron level will get you 200 credits, whereas an onyx level commendation can get you up to 3,000 credits.
Complete daily and weekly challenges. You can view the daily and weekly challenges under "Challenges" in the pause menu. Many of the challenges are straight forward and require you to do things like kill a specific number of enemies with a particular weapon. Completing a set of daily challenges can get you anywhere between 2,000 and 4,000 credits. Weekly challenges are usually worth about 1,500 credits.
Stick with teammates. In "Halo: Reach," you're more likely to get kills and credits if you target enemies that have already been wounded by someone on your team. In addition, you can earn credit bonuses for assisting in another teammate's kill or avenging a teammate's death.
Play SWAT games. SWAT is a Matchmaking gametype in "Halo: Reach" that is similar to a typical Slayer (or Team Slayer) game except there are no shields and every player starts with a magnum and DMR. The lack of shields generally makes it easier to get kills but, If you're a marksman, this is an excellent gametype to rack up a lot of credits, as head shots equate to a one-hit kill.
Play Multi-Team games. In Multi-Team games, six teams of two compete in Slayer and other objectives-based matches. Because this gametype is often fast-paced and frantic, you have frequent opportunities to steal other player's kills and earn credits. And, since Multi-Team games don't usually last long, you have more chances to win the "lottery" credits awarded at the end of each match.
Play Gruntpocalypse. Gruntpocalypse is a version of Firefight where you take on a horde of grunts. Since grunts are small creatures with little armor, they can easily be taken out and are often killed with a single head shot from the battle rifle. By playing this gametype, you can easily earn 2,000 credits in under 10 minutes and can earn additional credits by completing commendations tied to the Firefight mode.