First and foremost, you want to separate your characters so they do not form a horizontal line. This is because his attack will attack in a straight, horizontal line across the entire screen. You can move your characters by holding down the "L" Button when you are picking commands. This will prevent both your characters from being hit at once. If both your characters fall asleep, you can easily be overwhelmed.
After you have done this, just spam Glacius and Flamma on him and he'll go down quickly.