I hate this guy and he's super annoying. Lemme just tell you something.
This guy is not about dealing damage. You need to heal, protect, and most of all TAKE AWAY HIS FOCUS. I'ts also needed to have a stun spell ready and use it a lot. Those are my tips on how to beat him. All armor that has high strength and vitality, you should equip to Roald and Veradux. Dux need instinct for healing, so equip the highest instinct and vitality armor onto him. Keep both on Phalanx and disrupt his focus as much as possible.
Those are my tips.