Focus on adds.
In the vid of the sword guy soloing this... He simply went in there and dpsed the boss down immediately ignoring all adds. Then he went around and killed the adds and black spheres.
I feel like that might be good to do since you don't have to worry about the spheres reforming continuously. As long as you can do it fast. Don know if the boss or adds get buffed too much with more players, cuz if so this may not work. Tether or melting point, then shotgun, sword, snipe stagger him down to oblivion I'm thinking.
Failed last week when trying to manage adds and him at the same time. Eventually room just filled up with tons of adds with the boss still alive and throwing endless blind balls. I think our gear sucked though