Get scav/arch/bio up to 550 and find a good place to run for mats (labor valley/yavin/that mando planet from chapter 14) then just run for mats and grind them out. You have to learn what the best paths are. There's one 5-minute loop in labor valley that refreshes every 5 minutes and yields about 30 of each of 9 different mat groups.

There's also loot crates in labor valley if you know where they are it takes 7 minutes to gather all 14 crates and each one has a small crafting bag and an item roll (usually low-level gift) and a possible med/stim roll. The crates refresh every 15 minutes, so you can grab them all in one seven minute run and thenQT back to the start and switch to PVP phase and do the whole route again. That way you're getting 28 craft item rolls (up to 10k each on the GTN) and 28 loot item rolls (up to 5k eack) every 15 minutes. That's 420,000 credits in profit potential for ever quarter-hour of grinding. You'll want a whole free 80-slot cargo bay to store the mats as you collect them, because you get all of them. Your probable actual earnings, presuming you GTN everything you find instead of vendoring, is in the 1.2-1.4M/hour of grinding, not including the time spent listing everything at the right price on GTN.