EDIT: Support for this game ended in late November 2019 and it appears that this achievement is no longer obtainable. I assume this is because you can no longer buy booster packs and instead are now simply given the full collection, making it impossible to get the cards from packs as originally intended.
https://mtgduels-support.wizards.com/hc/en-us/articles/36003...
You have a chance to get a Mythic Rare card from every booster pack you open. Packs cost 150 gold each, which can be acquired by winning solo duels (5/10/15), online duels (30), from daily/community quests (40-120), or by buying gold for real money on the Xbox marketplace.
There are 10 different Mythic Rare cards. Two in each color- one planeswalker and one creature.
NOTE: With new sets having been released, there are now more than 10 mythic rares. This solution was written when there was just the Magic Origins set available.
Black: Liliana, Heretical Healer and Erebos's Titan
Blue: Jace, Vryn's Prodigy and Disciple of the Ring
Green: Nissa, Vastwood Seer and Woodland Bellower
Red: Chandra, Fire of Kaladesh and Avaricious Dragon
White: Kytheon, Hero of Akros and Archangel of Tithes
This achievement will pop upon acquiring any of these 10 cards from an Origins pack.
There are two additional achievements for collecting Mythic Rare cards.
You need to collect all five planeswalkers to unlock an achievement:
You need to collect all 10 Mythic Rares to unlock an achievement: