Not sure if you are still trying but what worked for my group was all 3 focus on adds. As soon as theyre about to die just shoot everything you got on the boss. As soon as more adds/ blights spawn kill those ASAP then focus on boss. Rinse and repeat. I was 303, others were 296 and 297. When we finished it we had a minute and a half left. Still is a very hard encounter. Good luck man