Elemental Monster: Online Card Game

I unlocked the trophy when I got a Rare card, not a Legendary one. I have heard of someone unlocking this when obtaining a Super Rare card. There is no sure fire way of unlocking this, it's possible to unlock a Rare card or greater before beating the Demon King the first time but it's more likely that you'll get it between the two fights if you're lucky. If you're unlucky you won't get one playing single player or multiplayer for a long while.