This trophy can be obtained anytime after clearing Chapter 5 when you gain access to the intervention quest "Where Art Thou, Smoochy-Poo?" and Warrior of Light Champion Medal. Make sure you have equipped Warrior of Light in the Champion Medals section of the main menu and that your Champion Gauge has at least one charge before starting the quest. Summon the Warrior of Light when the Princess Goblin is within range of being killed.
Credit to Eternal_Rach