Instructions
Play through the "Fire War" quest until you reach the "Battleon Burns" section. Complete this section until you defeat Jorhath. After this battle, equip any weapons you have that are based on either water or fire. The next fight involves a foe that is weak against both elements.
Enter the battle with General Varax. His health is indicated by a red bar on the lower right side of the screen. Attack him with all members of your party using melee attacks to wear him down.
Use the "Kathool's Wrath" spell to drain a significant portion of his health. You can only use this spell if you're wearing the Kathool Adept armor. If you don't have this armor, keep using melee attacks to diminish his health. Use a healing potion or the "Turn" spell to restore lost health. Continue the attack until he is defeated.