Easy Credits
If you're not one for going into the main game to get credits, firefight is your best option. Most of the little games there let you rack up a lot of points in little time. Gruntpocalypse is probably the easiest to get points in, as its one shot, one kill with them. Might as well set it on Legendary if you're a good shot, seeing as you can pop them off just as easily on that difficulty as all the others.
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