The best way to defeat him is by staying away from him for the most part. Use more powerful moves on him when he's attacking other characters. Make sure to switch to Katara and use the group heal if you notice anyone's health going down. It's very important to keep Katara alive. Then, give him all you've got when he kneels down. He only does this a couple times, but make sure you get to him and attack him the most then.
*Half way through he goes all inferno and has fire at each end of his staff that he'll swing around and just walk. If you're too close it'll hit you. Be aware of that.
*Also try and use the little statues or whatever they are on the platform to help block his floor fire attacks. stand behind one and you won't have to keep running around. **Don't get too close to him while he's throwing his fire otherwise he'll just quit and go after you.