If you want to beat Riku IV, fill your deck with as many 9 cards as possible and a bunch of 0's. Wait until he uses a card and break it with a 9, hitting him or just keep slicing away with the 9's. Save the 0's. When Riku uses a sleight, use a 0 card. This can be especially helpful with the Dark Aura move. Keep a large amount of Cure's in stock and use them when Riku reloads. Omnislash is also very effective.