Go to the Guard's Corridor and kill Flame Core Guardians and the Frost Core Guardians. They have a chance to drop a green item called Frost Energy and Flame Energy. These mobs are found in the middle of the Guard's Corridor area. Take the Flame energy into Mystic Altar and use it on an evil ice spirit (The one that is summoned when you take the water). Take the Frost energy into lvl 50 Ice Dwarf and use it on Thynos - the fire boss at the end.