Git Gud. Seriously though, in the rooms before the boss kill blights then wizards the snipers then the rest...in the final room clear adds then focus on the boss...when new adds spawn kill them then focus on the boss...I failed the first time but finished the second with 3 minutes to spare.

Screw the ads. Get your swords out. All 3 of you rush the boss and swing away on him until you die, then do it again and again until he is dead. Then you will have anywhere between 5 or 4 minutes to clear ads. I do this strategy everytime and have about a minute and a half left on the clock.

I have two manned it both times. A warlock with storm caller and a warlock with sun singer is key. As soon as you enter boss room have sunsinger tank up on boss. Do as much damage as you can with sword until you die. Resaurect and continue to dps boss until you die again. When you re spawn have storm caller clear ads while you kill blights. You're super should be up again to recycle damage on boss again. After boss is dead kill the three blights that spawn and clear enemies. Voila!! Black spindle. Are you on ps4 by chance?

Clear all adds and blights. Do not shoot the boss until you do. Then blast the hell out of him. New adds spawn, repeat. Save supers for adds, use heavy and snipers on the boss. If the boss is real close to dead when the third add mob spawns, focus fire on him until he is dead and clean up the adds. Communication is key. Try to stay alive because those precious seconds waiting for revive are worse than ducking back into cover for four seconds to regenerate health. Will of light on any weapon helps.