Sulfurious
Sulfurious is pretty much the dragon you fought in the tutorial. However he is a severely beefed up version of that dragon. He has no special tricks up his sleeve like Kang Kung's falling debris or Lich's minions and their sacrificial healing. He is just a straight up tough Battle.
Sulfurious and his whole stage consist entirely of fire enemies. Bringing a good team of water element heroes will be ideal to take the least damage from all the fire attacks. Before Sulfurius, a heavily-armored mini boss of two minotaurs will Battle you. These enemies are particularly strong and should be dealt with care. The actual fight with Sulfurius is straightforward and will rely on your team synergy.