Blood of Bahamut

"The game is set in a city built on the back of a great beast known as a Gigant. The city suddenly comes under attack and protagonists Ibuki and Yui must defend it as well as other cities that were built on Gigants that are awakening. The story features seven main characters in total. The Gigants are sentient, and will also attempt to defend themselves, taking part in battles. The Gigants include Bahamut, Ifrit, Shiva, Fenrir, and Gilgamesh, many of which are also summon monsters depicted in Square Enix's popular Final Fantasy series."

"Blood of Bahamut is an action role-playing game in which up to four players battle giant beasts that are featured on both screens of the DS. In order to defeat a giant, players must first destroy its protective armor, then attack its "core," and finally attack its other body parts. The giants can summon smaller beasts, which the players must battle simultaneously. Defeating a giant's body parts and the smaller beasts will grant players "materials" which can be used to produce powerful weapons. The game contains over 130 missions."

Source: Wikipedia