u must have 0 cards and cure cards too let riku use Dark Firaga and press left or right tap 2 keep doing this untill he getting run out of cards if he use dark aura use 0 card to card break him if he reloading cards use cure cards fast that all i have to give u tips hope this is help
to kill fake riku try B+B+A in Dark mode it deal damage keep doing this untill u kill him when he use Dark Aura use 0 any cards also use enemy cards it help u too
