There are ways to defeat him,but you need some preparation to survive Riku's attacks,because if he goes berserk,his attack power is increased.What you need to fight evenly with him is:
- Critical Plus (Optional)
- MP Haste
- MP Rage
- Guard
- A powerful Keyblade, Divine Rose, Pumpkinhead are recommended because it's ability to deal critical hits.
- High level
- Sonic Blade, Strike Raid, and/or Ars Arcanum.
- A lot of Ethers,and maybe one Elixir
- Cura
- Aerora
After all preparation,begin the battle using Aerora,and start using Strike Raid,Sonic Blade,or Ars Arcanum.Try to make combos with these skills,because each skill can deal more than five hits,plus the final strike.If your MP is low,use Ethers or an Elixir and keep using Strike Raid,Sonic Blade,or Ars Arcanum.If you are using Ars Arcanum,do not continue your combo with the additional attacks,because you are vulnerable that time.During the duration of using these moves,you are invincible.I would advise you to let Riku hit you with one of his attacks because his attacks are damaging,and you will be damaged a lot,but your MP will be restored greatly too,thanks to MP Rage.Save your Ethers and Elixir until Riku goes berserk during second phase.