Where is best place for Normal Credits

So, we cleared challenge mode (the combat way) and now its time to optimize the gear, my issue is that the new credits.

With the old system the base cost of a reroll on a yellow was 31 phoenix, a daily challenge mode gave 50 as a reward so the base cost was 60% of your daily reward.

Now the new system has a base cost of 106960 credits for the first reroll and you only get 17500 from a challenge mode.

So we have gone from 1.4 base rerolls per challenge mode to now needing to run 6.2 runs to afford 1 reroll, and the scaling gets worse for every subsequent reroll.

So my question is has anyone found an efficient way of generating credits since the patch.

(edit) My max credit run at the moment is General Assembly and its netting about 250k an hour.