Kingdom Hearts Re:Chain of Memories

You can only obtain this trophy through Reverse/Rebirth. Unlike Sora's storyline, when Riku does a card break against an enemy with the same number card it initates a duel challenge which you can start if you press . Each duel has a time limit depending on the difficulty of the heartless, the heartless will throw out random attack cards and you must respond with cards of more value. Sometimes you can get away with mashing out your deck to clear it and a lot of times you'll be a in a world where your cards are just too weak. Grind it out when you can, it should also be noted that you can get a lot through boss battles for Riku.

Grinding Out Duels
If you want to get it in a more effective matter you can do so against Black Fungi, which can only be encountered through the Dark Room Map Card. Follow these steps for an easy grind:

• Encounter a Black Fungus and kill off any extras on the field
• Once there is only one left wait for it to harden so that it takes no damage
• Black Fungus usually always use a 7 card so queue a 7 card up for when you initiate the duel
• The rest of the Black Fungus Deck is weak so you should have no problem winning and because it's invincible it won't get hurt.
• Rinse and Repeat

It can be a bit tedious but it gets the job done, if you want to guarantee more winning duels do this in Neverland where you have a relatively strong deck or in Castle Oblivion.