Zack will encounter three foes at once in this fight. Focus on defeating one of them at a time. Magic attacks will inflict quite heavy damage on the machines, but standard swords attack or critical hits will work as well.
Be sure to equip Zack with any accessories that will nullify Poison status to guard Zack against the Shot Machine's Poison Gas. Nevertheless, high Magic stats and Thunder materia will be the best way to achieve quick victory.