Ok, calm down and relax. If you are on expert mode it will take some time. Here is what to do.
Normal Mode: Make sure you got dodge roll as a support option and equip it. And have lots of potions handy. The way to beat him is attack the heads. Use the triangle button if you got a lock on Cerebus (that is the dog's name) and Donald and Goofy will automatically attack him ( you know they are about to attack becaues a little exclamation mark appears above their heads). BEFORE you fight, go to your customise screen and adjust both Donald and Goofy to make everything frequent, Except potions of item using. Make that" sometimes", don't worry, if Sora goes into red zone health your friends will toss you a potion, or Donald will cast cure on ya. Sora should not use any magic against Cerebus, only physical attacks. Donald will handle the magic, you save your MP for using cure. And believe me you will use it. Allright here is the game plan, when you begin just attack the heads. Cerebus will shoot fireballs at you, RUN AWAY. When it stops Cerebus will put its heads down, ATTACK it, get alock hit triangle and go for it. Be warned he bites back. Eventually he will spit evil from Hades out of the ground. That is what the large black circle under your feet is. Use your dodge roll. He uses these same tactics until you beat him. And if he ever rears up on his back legs watch out he is using a massive shock wave, so jump to avoid it.
Expert Mode: Everything is the same as normal mode asis my advice. However I personally left Olympus and leveled up before I fought Cerebus. And it was a lot easier to beat him after I was stronger.
