I have to admit, the first Riku was tougher than I thought. I entered battle expecting to murder him half-assed, but I went into critical health a few times.
My deck sounds the same as yours. Even though I'm not up to him yet (one world away), what I should recommend is use more sleights that have reaction commands. If he cancels those too, then just wait until he uses a card and cancel his with either a sleight or a combo of 3 cards. Tossing in a magic spell shouldn't be bad, thouh summon cards would "probably" get cancelled.
Lastly, use cure cards to cancel his move, or use Goofy/Donald in a sleight, then throw in two cures for a Cura and heal fair health. Just never use a cure card first in a sleight unless you have 4 or more cure cards in your deck, which you shouldn't need this early.
Don't want to write an essay on you, but I hope this helps.