I got a lot of problems with this boss, this is how I finally beat him:
Equipment:
Healer:
Healing bracelet
Tanky:
Durandill
Great Defense ring
Let everybody else die except tanky, how faster they die the better.
Use invulnerable on tanky, every time it is useable. I had to put upgrade points into this spell to let it has -3 sec cooldown.
Use counterspell when the boss uses the dark cloud spell on Tanky.
I have most of my points into great heal, which means it auto heals a lot
when just equipped. But in case Tanky needed some health I would cast
this on him.
And for most of the time I would use Meditation.