Like the text says, Earn 1 Million Credits.
This is Cumulative and doesn't go down. So you don't need to have a Million Credits at the same time.
Some Credit earning tips: There are Credit Boosters. Some free boosters which may drop on daily Login, but event or purchased booster will help.
While Farming missions will help, doing the harder content can easily earn more Credits per hour, assuming you can do the missions. More players in a team can create a bonus, while Alert Mission will already have a high credit reward. (It's worth noting that completing a set number Alert Missions and waiting for a timer will earn another Achievement.)
Of course, selling your useless items will help.