After you beat Darkside on Distiny Islands then you have to fight Riku...

A good way to beat Riku is to build your deck's CP to 900-1000. Also try to put some high attack cards around 9-7 that level.

Also put some Cloud summon cards to have some high speed attacks and also strong ones. Next, try to put Jafar, Vexen, that other enemy card that enemies from breaking your cure cards.

Finally, put 5 Cure cards.

My strategy was kind of weird. My deck had about 6 zero cards, 6 nine cards, about 15 eight cards, 2 seven cards, and 2 cure cards level 7 and 9.

As soon as the battle began, I switch to my zero cards. I ran around like an idiot dodging Riku's attacks.

Then every time he'd use a sleight, I would break it with my zero cards. He will recover his lost cards with ethers, so try to card break those.

When he stops using sleights, attack or heal while he's reloading.

Without any cards in Riku's deck (or a small amount), he's as easy as Hard Armor.

As well as having a good amount of CP, you should have the Max HP just in case you get bombarded by Dark Aura. The max is a line of green and blue (which I think is a rip off). I think you should space out your zeros and high attack cards, because he doesn't use Dark Aura in an alarming number. However, keep your Cure cards all together so you can sue their sleights if you get bombarded, and because they're easy to spot in a bunch. PS I haven't beaten him yet either, but that's because my CP isn't high enough to a full deck.