The Djinn is probably the easiest Boss in the game.
Lvl: 7
HP: 600
Atk: 20
Def: 8
Weakness: Ice, Water
I defeated him with 4 Dual Wielding Freelancers. I gave Arc the Cure LVL 1 spell for healing just in case. Just keep attacking unless he is going to be able to kill one of your character with 2 straight blows. Keep track of his damage per hit and double it to see if he can attack one character twice and kill him. If so heal but if you have the best armour available you should be fine.