Due to its nature, it is only possible to earn this feat with a new file. Even if one beats New Game Plus within 90 minutes, the Feat is not granted, due to play time being transferred over to New Game Plus.
In early versions of Shovel of Hope, a certain cheat code would create a clear file with every item. If used to start New Game Plus, the cheat flag would disappear after beating Plains, making this file able to complete this feat in New Game Plus. This exploit has been patched out.