Rogue Galaxy

The earliest chapter you can obtain this trophy: Chapter Two

Near the end of Chapter Two, you will face the Mud Whooper boss. Refer to The Jungle Planet for strategies to defeat the Mud Whooper. Once you have defeated this boss, you will gain access to Toady, who will be able to synthesize two of your weapons and turn them into a new, more powerful weapon. The criteria for Toady to be able to synthesize two weapons are the following:

• Both weapons must be of the same type. That is, you cannot combine two different kinds of weapons to make a new one. For example, you cannot synthesize one of Jaster’s swords with one of Zegram’s swords. You must either synthesize two of Jaster’s swords or two of Zegram’s swords.
• The weapons’ skill bar must be maxed. If you check the weapon’s stats in the Allies or Item tab in the Menu, if the word “MAX” is present beside “Skills”, the weapon can be used for synthesis. In order for a weapon’s skill bar to be maxed, you must continuously use it in battle at least 15 times.

Once you have two weapons that fit these criteria, enter the Items tab in the Menu, select Toady and press “Combine”. Combine both weapons, Toady will synthesize them into a new weapon, and the trophy will unlock.