Upon reaching Chapter 3 of the main story, Zack will receive a promotion and unlock Materia Fusion. Accessible from the main menu, this allows you to fuse two Materia together to get a new one. If you wish to get a lot stronger, then you will need to utilize fusion, as it is one of the only ways to get stats on your Materia. For the purposes of this trophy/achievement, though, you only need to use Materia Fusion a single time, without worrying about the result. For more information, visit the How to Unlock Materia Fusion page.