I beat Yiazmat without Espers, it took a total of 17 hours to kill him. The best combo to use is a tank and mage combo. Cast berserk, hastega, protega, shellga, bravery, lure, and bubble on your tank, use the white robes on your mage. Have your third character use just a few spells to help you out and then just melee Yiazmat the whole fight. Also equip your tank and 3rd character with Maximillian and the Magepower Shishak, for accessories just use the hermes sandals and the ring that lets you cast spells through reflect, for the tank just use the bubble belt, and your third character gets the same as the mage. Also make sure you have 3 windbreakers, when Yiazmat uses Cyclone you can quickly equip the windbreakers and cut the damage in half.