DRAGON QUEST XI: Echoes of an Elusive Age

To unlock this trophy you need to forge the Supreme Sword of Light on your Fun-Size Forge. This is only obtainable in Act 3 (Post Game) by following the below steps.

1. Progress the post game story until you unlock the trials found at the Zwaarsdrust underground ruins. Choose to partake in and complete The Disciple's Trial, making sure to collect the Phial of Lumen Essence found in a chest at the Eerie Valley - Pheonix's Nest. It's also advisable while in this map to pick up an Orichalcum from the mining sparkly spot. Complete the trial and select "I wish I had a super powerful weapon!" to receive a recipe book for the Supreme Sword of Light.
2. Zoom to Gallopolis to reacquire the Forging Hammer. This can be obtained by winning the Platinum Cup horse race on easy difficulty.
3. Make your way to Mount Huji and find Miko in the Caldera - Lower Area map. Talk to her and when requested give her the Lumen Essence you found earlier. After watching the scenes she will give you the Crucible Key.
4. While in Hotto, speak to the blacksmith with the pink speech bubble in the west area of the map. Sell him the piece of Orichalcum you collected earlier and he will mention using it to forge an awesome weapon. Leave town, rest a night and return to buy the Sword of Kings at the item shop behind the blacksmith.
5. Farm another piece of Orichalcum. This can be done in the same location as earlier or you can visit The Battleground. There is a sparkly spot location for this behind the Ultimate Key door on Level B8 or you can obtain one from the exact same spot you did in Act 2 on Level B9.
6. Forge the Sword of Light at the Crucible on Mount Huji and then the Supreme Sword of Light on your Fun-Size Forge.

You should have most of the other required materials to craft the Sword at this point in the game. The only one that may pose a problem is the Crimsonite which can be obtained by trading Serpent Souls to Krystalinda in Sniflheim. Serpent Souls can be stolen from Royal Reptiles at the Ruins of Dundrasil.