Invoke a summon at any point in the game to earn this trophy. When you can do this can vary - if you obtained some of the preorder summons (say, Cactuar), you could potentially use this as early as the fight with The Huntsman at the end of Chapter 2. Normally, however, you’ll be given an orb of Ifrit Materia at the end of Chapter 3. Equip it, and in Chapter 4 during the fight with the waves of enemies in the Sector 7-6 Annex your Summon gauge should start filling. Just summon Ifrit when it’s full to earn this trophy.