It is indeed "endurance" fight. Took me 2,5mins instead of the usual less than 1min to kill boss. It's more of priority learning. Once Manni channel the portal you need to shift priority to adds, then back to Manni and again adds and Manni. If I remember right he makes 3 portals that will spawn adds during the fight.
If it wasn't for the ghosts i wouldn't have survived. i used burning embers skill which healed me a lot whenever i killed one of them, usually in one strike.