Riku IV is said to be the hardest boss in the CoM. Here are my tips:
Attack constantly with high leveled keyblade cards.
Put in some level "0" cards. (I reccomend level 0 Kingdom Key cards)
Don't use magic.
Don't use sleights, your sleights will waste your cards and be broken by Riku's "Dark Aura" sleight.
Break "Dark Aura" at all cost.
Try breaking all of Riku's sleights before he could use them.
Use Oogie Boogies' "Regen" if your HP is low.
Use "Curga" (this should be the only magic you use)
You might just need to train more before fighting Riku. If you need any more tips, don't hesitate to ask.