Start by Dispelling his protections and using Haste and Protect on your party. Also, use a large chain of Quickenings to lower his HP by some, but save some MP for later. He likes to use Slow and Disease on you often. Get rid of it with the proper spell or item. After Hashmal loses three quarters of his HP, he uses a very powerful attack called Quakeja. To take absolutely no damage from it, cast Float on your party before he uses it. He will only use it once. Because he is an Earth type, he is weak to Wind. Shoot Aero at him as much as possible. Keep everyone above 50% HP and keep up your Aero to win quickly.