Halo Wars 2

To start, Blitz decks consist of twelve cards. In-game, you will only draw four cards at a time; when you play a card, you draw a new one. Cards are not single-use, though, so you could potentially draw the same card a number of times depending on the luck of the draw.

Overall, this achievement is most likely going to come naturally as you play Blitz matches for all the other Blitz-related achievements in the game. There is no alternative, expedited strategy involved here. Just play Blitz, play your cards, and, if necessary, recycle cards you have already played to increase the chance of a new card appearing in your hand. As soon as you play your twelfth card, the achievement will unlock.