Djinn is an Air-elemental Phantom, which means Earth attacks are effective against it. It uses two moves: Lightning Bolt, which is a single target Air-elemental attack, and the more dangerous Thunderburst Lv. 3 which targets multiple parts of Ashley's body. A good strategy is to cast Soil Fusion or equip a Gnome Emerald to self or weaponry, and equip the Djinn Amber to shield. Also, Phantoms are more susceptible to Blunt-type weapons, so use that as well.