As a PvE event, 5 heroes versus the boss - I guess you better take 2 healers, 1 tank and 2 dps... or maybe even 3 healers, hence he has violent aoe spells. Heretic & Doctor, Warrior, Phantom & Magician would be a good combo I'm guessing.
You'd have to spam stun effects and keep the damage coming. While that is being done, the tank will get trampled by his "20% from total health damage" which MUST be healed. At the start of the fight an AoE stun will be fatal if all of you are near him and if you are too far, you will get infected "dealing damage over time, reducing armor"
The bottom line is, while around the boss - your mana regeneration is -200%. You got two choices here. To get Priest, buy Wizard's cane and Refine it so the Priest's mana regen+Wizard's cane would negate the -200% reduction with +225% mana regen. Or each one of you to buy Holy Orb and get 0% mana reduction from his passive.
Its fun and challenging. And not even close to easy.