The best way to kill Metal Slimes is to use attacks that inflict critical damage. The hero's Lightning thrust ability performs a critical attack on these enemies. Another method is to use Falcon Blades, which allows you to attack twice -- this theoretically doubles your chances of inflicting a critical blow.

Make some elevating shoes and set them to your hero. Wander around in the Slime Mountain and kill all metal slimes you come across. Yes, it's hard, but here's how to do it efficiantly. Make sure to use weapons that are capable of using critical strike attacks. Heres the best choices that I have found useful with the most success rate:

Hero - Metal King Spear Lightning Thrust
Yangus - Conquerer's Axe Executioner
Jessica - Poison Needle Assassins Stab
Angelo - Odins Bow Needle Shot

Most of the time, your hero and Jessica will hit with better succession, while Yangus and Angelo can handle the lighter portions of the slimes that appear.