To unlock this, the Glittering Prizes, and Show me the Money for 10k, 100k, and 1mil credits is to play the game a lot.
The best ways to get credits is the Co-op Challenge with a friend or the Golem Challenge if solo. These two modes should help to cut down on que times for faster games.
You can also change the difficulty to make it slightly harder but give you more XP and credits per match. You will have to beat each challenge at least once to unlock the next difficulty.
For the lower levels certain strategies work like using a Soul Cauldron spawner as a tank and using a few Surges of Light to heal with Stone Weavers as your main damage to steam roll though the fight.
Other tips that will help you out considerably as you play the game is to only spend money on unlocking cards you need. After unlocking a card through its quest from the Card Vendor you can find cards by winning matches if you can wait.
When ascending cards to a higher tier for more charges only use low level dupe cards such as Echoes of the Future to level up the cards. Since the card can only be used once every minute and does not stack its the perfect dupe to use to cut down on costs of upgrades.
Clear out the weekly and all the daily quests for some extra cash each day.
Lastly if you want to purchase Crystals in-game you can buy Premium Time in the Shop to give you an additional 150% for days. When you can speed run the crystal difficulty that is a total of 350% extra credits.
Using my code below in the shop under Redeem Bonus Code will give you a starting 1000 Crystals. I recommend using it to unlock a skin for another achievement. If not it would give you a days worth of premium time.
5257360