Then I believe you mean Miguel as the boss, right?
Yes itīs a very difficult battle in your first game. In the new game+ itīs easy.
(SPOILER)Just remember not to have any more charachters innating black than Lynx/Serge and have daemon charm equiped on him.
Have a lot of healing elements like heal all etc. and give black elements to all charachters in your party.
You know when heīs about to use his most powerful attack when he casts weakminded on some of your charachter. Then heal fast and revive if needed.
Try to cast as much black elements as you can, but it can make him attack even harder. I tried to attack with weapons and when it feeled safe I used elements.
Just try to keep all your party alive and secure their status.
White plate would be very handy, but itīs onl available in the new game+ in this point of the game.Itīs going to be difficult, but even I get it through, and Iīm not that good. Patience is needed. The force will be with you! >D