This guide is to help with finding and killing the Gold Mecha Robo in the Ancient Factory once you gain access to the Mechat and are on Disc 3.
Finding the Gold Mecha Robo:
While Nahbein does present a pretty solid bit of help, he doesn't define how to find this guy. If you are on Disc 3 and have travelled to the Ancient Factory, chances are you may not find the boss. Why? Because you need to complete two other tasks to have this boss appear:
1) Find Mystery Part Un, Deux and Trois to forge the Final War Necklace.
Mystery Part Un - Head to Alumaru village (the Gorgo village) and travel into the cave where you fought the village's evil chief. It's in the caverns behind the house.
Mystery Part Deux - Sea Cube boss battle.
Mystery Part Trois - In a chest on the Western Deserted Island (Mechat access only).
2) Find Mystery Part Alpha, Beta and Gamma to forge the Ultimate Weapon Bracelet
Alpha - Found in Mural Town on the floor
Beta - As you reach the end of the Ancient Factory during your playthrough of the game, you will come across a circular room filled with Double Lance enemies. Defeat them all as they come to life and you'll reveal a chest.
Gamma - In the Mecha Base High (the one hovering above the water in the sea) on the 1st floor. It's in a treasure chest.
Got all six items? Head to Jibral Castle and find the funny-dressed scientist/professor in the basement's lab. He will create these two items for you (also unlocking two additional achievements) and now the Gold Mecha Robo is yours to battle.
Fighting the Gold Mecha Robo:
This guy's a pest. He has incredible agility so he'll have the opportunity to destroy your hit points fairly quickly. It is best to face him with a Lvl. 99 Shadow for each character. Your Skills are set to your preference but rely heavily on Endure, Resurrection, Double-Cast, Double Strike, Absolute Counterattack, and give everyone magic abilities with a primary focus on Black and White magic. My character levels were only at Lvl. 50 so your Shadows need to be maxed out.
If you have accessories like Hermes' Shoes (first strike) and Agility boosters, use them on your Barrier and Support magic user. This character will be able to get out first and Double Cast Quickus and Shieldus on the party to help block the sizeable attacks being thrown your way.
The rest of the party should be casting Flarus or Waterus using Double-Cast, while your strongest White Magic user should cast Zephyrus and Previve whenever he/she can. If you have a weak Black Magic user with strong Magic Sword abilities, utilize the Flarus Sword and Waterus Sword attacks.
After you establish yourself well enough to handle the attacks, heal constantly and keep the magic attacks coming. The boss' hit points aren't exactly high enough to render the battle impossible, but it'll help to play it safe.