You can unlock this in the first world, Olympus by finding the Water Cufflink or by having the DLC Keyblade from the Amazon pre-order: Dawn Till Dusk. The cufflink lets you use waterza and the keyblade lets you use firaza. Once you have the accessory or the keyblade, simply fill up your three arrows above your command bar by using the magic specified and it should prompt you to hit
to activate your grand magic and earn your achievement!
Water Cufflink location:
When climbing Mount Olympus after saving everyone in the city, you'll get to a flooded river. After defeating the heartless, the water level lowers revealing a chest. Backtrack down the riverbed and open the chest for the Cufflink.
I've attached a video giving you a visual on where to find the Cufflink. It's at 0:24
(Credit to underbuffed)