My brother was having the same problem with Chernabog. He had no time to heal between the fire waves, and eventually they killed Sora. I was on such high levels (about 70's) that the fire attack never really hurt Sora, but it can be very damaging on low levels. I decided to help my brother out, and although the battle was quite hard with his low levels, I still managed to defeat Chernabog on my first try. I can't remember if summons work when you're flying, but if they do, summon Tinker Bell. I just had Sora cast Aero/Aerora/Aeroga on himself to protect him from the fire waves, and then keep tapping the shortcut buttons for Cure/Cura/Curaga and hope that Sora can heal himself before a new fire wave interrupts his spell casting.