Update: People have reported that they have unlocked all cards prior to the challenge starting and were awarded this achievement.
This is for those that have all the cards and don't get awarded new packs including the DLC.
This is for those that have all the base game cards and don't get awarded new packs but don't want to buy the DLC.
This is for those that want to contribute to the challenge AND get credit but not lose all their cards/made decks.
If you bought the collection unlock DLC for the base game then you'll need to buy the expansion and unlock a expansion pack but if you bought both DLC's to unlock base/expansion cards then you my be out of luck. Uninstalling it does nothing.
The most simple way is to back up your save file to a external storage device and then deleting it to play the game from scratch.
Now for the harder way.
This was a method I personally figured out to work around a glitched Dead Rising 3 DLC achievement which actually worked for another achievement in a later DLC for the same game.
If you screw up something and delete your main save, please don't blame me.
Go to "My games & apps", press the start button over Magic 2015 and select "Manage game".
Move all the way to the right and find your gamertag, press A.
Select "Delete from console". ("Delete everywhere will permanently delete your file, don't pick that.)
Now go to the systems "Settings" and select "Network" to disable your internet connection. (Or just remove your ethernet if you aren't using WiFi)
Boot up the game real quick while offline,play through the tutorial and then go to options and do whatever you need to create a save file, going into deck edit and taking out a card and putting in another land is a option. Once that's done, quit out.
Reconnect to your WiFi connection (or plug back in your ethernet) and boot the game again.
After you Press A at the main screen it will ask you to select either brand new local file (This console) or your old cloud file (Xbox Live). SELECT NEITHER!
Picking one will overwrite the other, instead click the bottom option "Stop syncing". You will still play with the brand new file but it will not overwrite your old one.
Play whatever mode you want to get awarded a booster pack and then open said booster pack.
Once you get this done, quit out and boot the game one final time to select your old Xbox Live save file and get your old save data back.
Hope this helps.