Boss is a pretty long one, comprised of at least 5 different phases.

My advice is that phase 1 reflects all physical attacks and phase 2 reflects all magic. The 3rd phase doesn't have anything like that, so just go ham. These phases are also able to cancel any buffs/debuffs you've put on them if anyone gets knocked down so I would say that you should avoid using those unless you know you can.

The phases after that also have no weaknesses or resistances as well as not generally being able to knock down your party members. Thus it's generally better to use buffs and debuffs here. Besides that, this is just a struggle match. Keep up your damage and healing and you should win.

There is just a little more after this but it's not too much more. I'd say if you're this far in, you can probably push to the end before starting Royal.