This trophy is simple, for me it unlocked at around rank 17-18 (Truetrophies seems to have a problem right now and it won't update for me but I have unlocked it).
At the end of any mission, battle etc your score will be divided by 10 and then converted into credits. EG A score of 5000 points will earn you 500 credits.
Simply complete enough missions and battles etc to earn 25,000 credits, it doesn't matter how much you spend so you don't have to have it all in the bank at once as it were.
I found I was getting the highest scores on things like:
Drop Zone: Completing objectives, capturing pods and defending them
Hero Hunt: Once you become the hero you can get plenty of kills so your combat score should be high on this one.
Cargo: Again returning cargo to base gets you high objective scores.
I found although I was getting a high kill count on things like Blast, there aren't any objectives to complete so my score was average on this one.
If you want to boost a little bit you can play the game on the companion app too which earns in game credits.
All in all a simple trophy which you will unlock organically just by playing through the game.