S few things to say.
-Tried New Alexandria all skulls on (Including Iron) two hours in and only had 80,000 credits. DOESN'T WORK
-Pillar of autumn with a lot of skulls on- On legendary this mission takes for ever. On Normal you fall 40,000 credits short.
DOESN'T WORK
I've tried so many to make these kinds of challenges work. None of them seem to work for me. I mean, I'm not very good at halo, but still.