has anyone optimized the best way to earn credits
Is matchmaking the best way to
is it even worth it to do firefight or campaign ones
You can checkpoint boost campaign ones and earn massive amounts of credits. But that gets boring quickly.
For FF and MP, play in matchmaking and just set up challenges to match your playtime and that will simply increase your credit gain. Especially good is doubling up on a Daily Challenge - set the same goal in custom challenges to get payed twice for the same thing!