Although Reach is my favorite game of the Halo series by far, I feel like credits need to be earned just a little faster. and the ranks take ages to go through, I've been playing almost forever and I'm still a Warrant Officer, almost Grade 1. Hours and hours of gameplay and I still get barely anything.
Try grifball but prepare yourself for a bit of possible frustration and you should be breezing through ranks in no time.
Based on your Halo Reach Service Record, I can safely assume that you do only have a Xbox Live Silver membership at this time which prohibits access to matchmaking. Then, there exists the following methods to obtain Credits outside from the fixed amount given from Campaign Commendations and Halo CEA Terminal codes.
  • Credits for time played
    • Constant amount of credits given for every minute played
    • Maximum payout after 10 mins
  • None Matchmaking Weekly/Daily Challenges
The payout for time played in Halo Reach will vary based on rank and here are some of the recorded credit payouts and as you can see, it's extremely low even at the later ranks.
  • Recruit: 50 Cr/10 min played
  • Legend: 312 Cr/10 min played
  • Mythic: 344 Cr/10 min played
Based on this, my method to rank up in Halo Reach with a Xbox Live Silver Membership would be to focus on completing none Matchmaking Daily/Weekly Challenges and to limit and to quit and then resume the play session after 10 min in order to not get credits wasted from time played.
I made it to General Grade 2 with a silver account. Check everyday for campaign related challenges (although there are many days where there are none) and farm the campaign commendations the best you can.
Based on your Halo Reach Service Record, I can safely assume that you do only have a Xbox Live Silver membership at this time which prohibits access to matchmaking. Then, there exists the following methods to obtain Credits outside from the fixed amount given from Campaign Commendations and Halo CEA Terminal codes.
  • Credits for time played
    • Constant amount of credits given for every minute played
    • Maximum payout after 10 mins
  • None Matchmaking Weekly/Daily Challenges
The payout for time played in Halo Reach will vary based on rank and here are some of the recorded credit payouts and as you can see, it's extremely low even at the later ranks.
  • Recruit: 50 Cr/10 min played
  • Legend: 312 Cr/10 min played
  • Mythic: 344 Cr/10 min played
Based on this, my method to rank up in Halo Reach with a Xbox Live Silver Membership would be to focus on completing none Matchmaking Daily/Weekly Challenges and to limit and to quit and then resume the play session after 10 min in order to not get credits wasted from time played.
This is cool, I didn't know there was a time related dynamic. OP, how long have you had Reach?
If there is a time related dynamic for campaign missions, it stopped working for me a long time ago. Ever since Brigadier rank or so at the latest, I've been capped at a consistent 378 credits for campaign missions which I can get after a few (perhaps 10) minutes of play but remains the same even if I finish. I just did the "House of Pain" daily challenge which took me 48 minutes but gave me 378 game complete credits. That was the case when I had a silver account and still is the case now that I have a gold one and I'm 2/3 of the way through Noble.
I played a ton of SWAT when I was grinding out to Inheritor. Maybe give that a shot.
If there is a time related dynamic for campaign missions, it stopped working for me a long time ago. Ever since Brigadier rank or so at the latest, I've been capped at a consistent 378 credits for campaign missions which I can get after a few (perhaps 10) minutes of play but remains the same even if I finish. I just did the "House of Pain" daily challenge which took me 48 minutes but gave me 378 game complete credits. That was the case when I had a silver account and still is the case now that I have a gold one and I'm 2/3 of the way through Noble.
The time related Credit payout for Campaign in Halo Reach is a payout for every minute played until you have played for a total of 10 mins and that it's extremely low. For me at Mythic, this payout is 344 Cr for 10 mins played (312 for Legend).

Based on some testing, the increase of this payout per rank is exponential at the later ranks and it will increase by 10% per level and based on the numbers you showed, this do also hold for the Noble rank (i.e. 378 ≈ 1.1 * 344).
OK, I understand better now about the max credits per campaign mission being reached at the 10 minute mark.
OK, I understand better now about the max credits per campaign mission being reached at the 10 minute mark.
An identical credit system is also used for all of the other none matchmaking gametypes like Firefight, Forge and Custom Games. Due to this, other gametypes can be played without losing credits for time played.

However, a slightly different system is used in Matchmaking which doesn't have the 10 min cap and do also have a bit better payout per minute which is dependent on playlist.
As an aside, I have noticed that the rules are different for non-Xbox Live accounts. However my son's offline account seems to consistently get an unusually high number of credits. I just ran through 'Nightfall' on co-op Normal with his (Brigadier Grade 1) account and another (Major) offline account. At the end, the other account got 1,296 credits but his got 3,321. His account gets much more than average credits for custom games as well.