Ok. Lots of heals. Fire. KILL IT WITH FIRE. It deals a decent amount of damage, while keeping you at a distance. If you have Fira, use that. It might take a while, but with fire boosts (Sudo Neku or Hebby Repp at that level), multiple firas or fires, and a couple of heals, you should be able to take him down fairly quickly, even as critical. I would not recommend the same strategy for Riku though.