Despite the manner that you have to obtain Lv.8 Magic for another trophy, this trophy requires you to attain the maximum form of each weapon. This is marked by two things. In the Weapon screen of the menu, the weapon will have a red name. You will see on the weapon screen eight weapon orb slots, which means that along with the those, you will need an extra orb to turn the name red. Once the name is red, this weapon will be able to be forged to the max. After that, all you'll need to do is find Watts and forge it.
Enemies in the Mana Fortress have a chance of dropping a weapon orb instead of one of their rare item drops. The ones to watch out for:
Glove Orb (x2): Wolf Lord
Axe Orb (x2): Fiend Head
Spear Orb (x1): Tsunami
Whip Orb (x1): Mace Menace
Bow Orb (x1): Metal Crab
Boomerang Orb (x1): Master Ninja
Javelin Orb (x1): Whimper
The last Sword Orb is unobtainable, as you don't need an orb for it. Instead, during the last battle, using Dryad's unlocked spell [Mana Magic] from both Primm and Popoi. This will give you the final Sword upgrade, the Mana Sword, as well as the trophy should you have everything else upgraded at this point. Make sure you return to the surface before you face the final boss to upgrade everything.