to riku is law it is very hard but u have to keep trying use the ultima weapon like u are if u have elixers or mega elixers put those in your item slots. when u fight him make sure u have sonic blade on. one of his attacks kept killing me befor i figured it out when he uses the attack when ur hp goes to 1 and no mp. you want to stay as close to him as possible when he uses that attack use sonic blade if you are to far away to use it get far away from him as possible and get ready to use a elixer. sonner or later he will start haveing dark ball things fly at u and he will be glowing blue thats when you dont give him time to respond and kick his as*!!! and when he makes all that fire come up around him do not i repeat do not use arc arcanium you want to only use your mp for healing and sonic blade. sorry if i spelt anything wrong
but if anyone needs help beating him just to say u kicked his butt post here that u need help and ill tell you how to beat him