As stated this only really works when you get Commendation awards.Iv been doing this for about 30mins and I only seem to score big when i get a Commendation.
So if you are getting low creds just keep at it untill you see an award for a Commendation pop up then just quit and save to cash in on the creds.
Once you max out said commendation then it's all over for the farming.
From what I can see the following Commendations can be grinded with the above method:
Splash Damage -
(Kill an enemy with a heavy weapon during the Campaign)
Walking Tank -
(Earn a multikill during the Campaign)
SpecOps -
(Kill a Specialist-class enemy during the Campaign)
Leadership Element -
(Kill a Leader-class enemy during the Campaign)
Super Soldier -
(Earn a Spree of any type during the Campaign)
Cannon Fodder -
(Kill an Infantry-class enemy during the Campaign)
and if you have any left overs after, you can use the DMR/Grenades to get:
Pinpoint -
(Kill an enemy with a Headshot during the Campaign)
Precisely -
(Kill an enemy with a precision weapon during the Campaign)
Nice Arm -
(Kill an enemy with Grenades during the Campaign)
enjoy