A lot of people have trouble with Djinn. He is, without a shadow of a doubt, the hardest boss in the game. But the secret lies in learning his attack patterns. He has four attacks: a lightning bolt, a ground wave, a blue arc-thingo, and a rush attack. His usual pattern is to use three attacks from his first three and then to use his rush attack. You have to learn to notice each attack and to avoid them all. I will outline each one, using Sadler to demonstrate, as Sadler's the only one really able to keep good distance and still hit for respectable damage with a fast attack and enough agility to avoid Djinn's attacks. Rumi's attacks don't always work, Fakyle is way too slow with his magic, and Kindi is just utterly worthless...slow, short range, and jumps like an elephant.
Lightning bolt: he will stretch his arm out towards you. Jump away from him, as you can't hit him during this attack or you'll take heavy damage.
Ground wave: he brings both of his arms in, then throws them out, releasing the ground wave. Jump over the wave and slash away. You can get in quite a few hits with this attack.
Blue arc: He raises his finger into the air and you see the arc spin for a second. Jump over the arc (this takes some timing but you'll get it down after awhile) and slash away. You'll get in a few hits.
Rush attack: When he lands, he will appear to do nothing...and less than a second later, he'll quick fly across the screen along the ground. Jump over him and land in front of him to get in a few hits.
You have to learn to react to all four attack patterns. While Djinn at first is a bruiser, once you learn his patterns he's a sitting duck and you can kick his ass without taking a single hit. Each time he flies to the other side of the screen over you, make sure you're somewhere more than halfway across the screen away from him. That way, his lightning bolt can't reach you, and you have more than enough space and reaction time to counter his three other attacks. But watch out too...he will occasionally mix things up a bit and use his rush attack early...so you should be especially aware of what his prep for that attack looks like.
If you need help with the Floating Garden, let me know.