Minion Masters

You will need at least 125 shards to unlock this achievement.
Shards can be obtained by completing quests, winning battles, unlocking them as a spin bonus with power tokens, gaining them from chests, unlocking them as a reward for getting certain achievements, or purchasing them with with rubies.

From the hub, go to "Deck Building," which can by visited by clicking on the bar across the top of your screen, or an icon in the lower right of your screen.
From there, click on "Crafting." Rare cards cost 125 shards to recruit. Press Y while hovering your cursor over a card to see if it's common, rare, legendary or supreme. Or alternatively, look at the card colors; blue = rare. Find a rare card and click the green button to recruit it. Doing so will unlock the achievement.

^{If you feel the need to downvote this solution, please comment why so I can improve it. I'm here to help you. Thanks!}