Riku is giving me a pretty bad beating. This is the fight after Destiny Islands, when he says something like "...in other words, your memory is a trainwreck...". I manage to get him down to half his last bar, but then he hits me with the move that takes up most of the whole screen (forgot what it was called).

He's using Dark Aura on you. That attack is devastating. If you want to stand a chance against him, make absolutely certain that you go into the fight with several level 0 attack cards. Once the screen begins to go black, use a level 0 card. Level 0 cards will cause a card break on both ends, immediately ending Riku's powerful sleight attack.

Stock your deck with plenty of healing cards as well. Again, the most important cards to place in your deck when dealing with Riku (or any boss for that matter) are level 0 cards. Try not to use such cards in your sleight combinations. If you do, you may not be able to retrieve them after recharging your deck.

Oh! And if you possess any Cloud summoning cards, bring them to the fight. Cloud is terrific against boss battles. If you can combine several Cloud cards, you can unleash his omnislash ability. That attack deals massive damage and allows Cloud to move across the screen rather than remain stationary. You may not be far enough in the game to use this, however.

If you have the resources to use the Holy ability, that also may come in handy. Holy is a light attack--Riku is a dark opponent. It's worth sacrificing the necessary cards in order to perform it.

That's about all the advice I can offer you.