Getting this achievement can be tricky because you not only need to get over 50 cards in your deck, but also survive and clear the game with such a huge deck.
Some tips to help you get enough cards:
- Get a card as a starting bonus, if available.
- Prioritize the skills that either give you gold (Gold Bar, the skills that give you additional gold for killing enemies or clearing a stage), or reduce the price of Data Store reset.
- When choosing side missions, prioritize the ones that give you gold (250 gold can get you several cards, which is better than a single card reward), then the ones that give you cards.
- Don't buy items or skill points in the Data Store. I'd also recommend not spending gold on expensive cards. It's usually more worthwhile to reset and get a couple of cheap cards instead.
- When defeating the enemies try to aim for Precision strikes in order to get the additional gold.
As for surviving, I recommend choosing a Hybrid deck (having to obtain 50 cards makes it nearly impossible to use a pure Attack/Defence deck) of a character you are most familiar with, and practice a bit beforehand.
Personally, I was able to unlock this achievement with Claire and her Hybrid deck.