This achievement will come naturally as you play the game but it does help if you have a Catalyst that makes enemies drop more credits. And I imagine a Currency Booster can help too.
Unless you come across a Danger Room Scenario with the Utility:Enemies are Wealthy
That will help you A LOT since it makes every single enemy drop up to 200 credits each.
Also since it is a danger room, even if you don't do the objective to progress, killing enemies gives you a slight percentage towards the objective and the higher ranked the enemy, the higher the percentage will be so it is advised to avoid high ranked enemies and go for the lesser mobs as they also infinitely respawn. I got at least 277k from a single Scenario.
I forgot to mention that Legendary missions are highly recommended too since you get a hefty amount of credits.
Also note that only credits that you pick up counts towards this. Getting credits from selling items doesn't count.