1) Equip Black Masks.

2) Start the battle with only one character then bring in others after his first Darkja.

3) Dispel his positive status ailments.

4) Keep up with Beserk and Haste when pummeling him with physical attacks.

5) Once he starts shifting his elemental weakness you can try finishing him off with a Quickening.

6) Alternatively, you can change your party's gambits so that their top action is to cast a spell like Scathe. There's a narrow window of oppurtunity to hit him with spells once he starts with his magick barrier and paling. He's easier to hit with spells than physical attacks.