The Riku Replica is fought on several occasions in Kingdom Hearts Chain of Memories and Kingdom Hearts Re:Chain of Memories. In "Sora's Story", the Riku Replica is encountered in the 7th, 8th, 11th, and 12th floor Exit Halls. In "Reverse/Rebirth", the Riku Replica is encountered in the 8th basement Exit Hall and in the facsimile of Twilight Town.
Kingdom Hearts Chain of Memories
When fighting the Riku Replica, Sora needs a few enemy cards: Jafar, Oogie Boogie, Vexen and Card Soldier. He will also need a few Magic Cards, Attack Cards (numbered 7-0) and Item Cards other than Potion. Equip the Jafar enemy card and start attacking. When that runs out equip the Card Soldier enemy card and use that, then when those run out use magic and other cards. Save the Oogie Boogie boss card until Sora is low on health. Another option is to start with the Vexen card and after being defeated, use the Oogie Boogie card or a few high-numbered Cure Cards, then use the Jafar or Card Soldier Card.
Without boss cards, Sora should stock up cards numbered 7-9, an Elixir and a Hi-Potion at least, two or three Stop cards (for the Lethal Frame sleight), two or three Cure cards and some 0 cards at the end of the deck (to break the Replica's attacks). To save CP, insert some Premium Cards in the deck.
During the fourth battle where the Replica will use Dark Aura several times before needing to reload, a simple and effective, yet lengthy strategy is to have 0 cards ready to break his sleights. As each sleight renders a card unreloadable, eventually, the replica will nothing but basic attack cards left to fight, leaving him an easy opponent to take down.
Use sleights when possible, as they are an effective way to cause great damage to the Replica (depending on which sleights are used). Keep bashing the Replica with Sleights, reload (using an Item Card) and heal when necessary.
As a final alternative, Sora will need different types of attack cards with high numbers to form the Sonic Blade Sleight. He will also need to put the cards in order to keep stocking cards. At the end of the deck, put an Elixir or one or two Hi-Potions. A Cure card is optional. If Sora doesn't have many cards that can make Sonic Blade, use Blitz. It is also good to have Cloud cards in the deck, though this is also optional.
Stock up three cards to make Sonic Blade. Try to get all the hits in, then keep doing this until Sora runs out of attack cards. Use an Elixir or Hi-Potion to recover all his cards. Keep repeating this to easily beat the Replica.
In any fight in both stories, the Replica's use of his Shadow Card will prevent him from using 0-valued cards to break the playable character's sleights, for having them getting turned into 1-value cards instead, assuming the replica doesn't prepare an even higher-valued sleight himself, he won't be able to break any sleights from his opponent as long as the enemy card is in effect.
Riku can use a variety of cards, from enemy cards to item cards. The only downside to this is that one cannot change the cards in his deck, but this is not a problem with leveling up and having to add CP to the deck: the game will automatically add the enemy card or the attack card to the deck. Also, while playing as Riku, try to use the King Mickey cards that fall. Using the MM Miracle, Holy Burst and Inverse Burst Sleights will help a great deal.
There are two ways to go about this battle. The first is to try breaking Riku Replica's attacks and enter into Dark Mode; all of Riku's attacks become stronger and faster so that he can hit and get away before the Replica attacks him. Make use of the Sleights available while in Dark Mode—Dark Firaga, Dark Break and Dark Aura.
The second is to wait until Riku gets hit, enter into Dark Mode, and then begin attacking. But beware of this route: as Riku will have to have a lot of HP or use the Oogie Boogie enemy card so that he can be healed. Use MM Miracle if you can to also help with healing. In the original Chain of Memories game however, as Riku cannot use The King card during the mission in Twilight Town, try to avoid damage as much as possible to avoid using the Oogie Boogie card too early.
While in Dark Mode, beware of getting hit or having Riku's cards broken—he will revert to his slower and weaker normal form.
Kingdom Hearts Re:Chain of Memories