Complete all Apprentice encounters.
Apprentice encounter cards are automatically added to your deck when you play on Easy difficulty. Like most encounter cards in the game, you must complete one to unlock the next in the series. You will not need to customize your deck to include these cards, as playing your next session on Easy will include the newly unlocked one. The quickest way to get your Apprentice encounter cards done with is to play Jack of Dust a few times in a row.
Apprentice encounters act like a game tutorial and their completion requires you to perform combat tasks a few times to prove proficiency. (There are 3 total encounters. Level 1=counter melee attacks, Level 2=counter ranged attacks, Level 3=use weapon abilities)
***Warning for completionists: This can be obtained without purchase of the Wildcards DLC. There are a few achievements that are obtainable and unlock from a "Title Update" DLC. If you want to avoid unlocking any Title Update achievements when playing the base game, Only play on normal difficulty, and do not complete any cards related to the Kraken or Elder Lizard. The dev has included some DLC encounter cards in the main game and they can be identified easiest by a maroon/red tab in the top right corner of the card with a gold monster head symbol.***
*If this solution helped you in some way, please vote positively. If you think it needs improvement, please let me know how in a private message or by leaving a comment.*