Play Matchmaking as much as possible.
Babel Outcast: Also do Score Attack there you get on average 1000-2000 credits per game.
Select Firefight mode, then go to "Game Options", and choose the "Spartan Settings". Select the "Base Traits" menu, and choose the "Shields And Health" option. Change the "Damage Resistance" to "Invulnerable". Then, set the difficulty to Legendary, and start a match. Stand near enemies, and let them attack you. You will get approximately ten credits per minute by doing this.
Set up a game of Forge with the timer set for one hour, and then end the game. You will get a total of 3,654 credits if you win the game. After reaching the "Captain" rank, you will get just over 4,100 credits for an hour long game if you win. Repeat this process as many times as desired.