This is a lot of credits and will take you a long time, but if you just go for other achievements (Gen 10, 100 Daily Challenges, etc.), you will earn this faster than you think. I unlocked this while I was at Gen 3 at the end of a match and had no idea I had that many, but I sold a lot of burn cards and completed a lot of challenges. I recommend using a variety of weapons and changing them frequently to complete challenges. Also, this should be pretty obvious, this is cumulative. You do not need to have 500,000 credits at once.
Buying burn cards and selling them counts and allows you to earn additional credits (for example, buying a burn card with 500 earned credits and immediately selling it for 100 credits to earn a total of 600 credits).