Equip Cecil with The Crystal Sword, Helmet, and shield, then equip him with the Adamant Armor and the Zeus Gauntlet (glove). Make Cecil use the CRYSTAL to transform Zemus to Zeromous. Use Edge's Spoon dart and regular attacks, then use Kain's jump attack or White spear. Make Rydia cast METEO, NUKE, and BAHAMUT. Make Rosa cure the party and use the WHITE spell to hurt Zeromous bad. Repeat this about 4 times and Zeromous will die.