It seems some of the pre-order keyblade specials give you the opportunity to get this early on. Just like the combo finishers you get for attacks, the more magic you use the more the buildup occurs giving you magic finishers.
*** Spoiler - click to reveal ***I unlocked this after completing Mount Olympus and having to fight the first group of heartless as Riki in the DarkWorld. Use his magic until Firaza appears as a finisher. Use that and the trophy will unlock.