For credit farming, there is no better place than The Index on Neptune. You will need at least 50,000 credits. If you don’t have them, we suggest going into a public squad on Akkad, Eris for a handful of times (or only once if you’re in a decent squad).
We recommend the High Risk option for The Index. Once you have the minimum amount of points required, you can bank the reward. We suggest only going for 1 or 2 rounds at a time and then restarting, instead of doing a long run. It will take less time, since enemies will keep going up in level every round.