Do the omnigul strike, in the first room when eris says "the hive hunger for your light" or something like that. Get to the right side of the room and nova bomb the thrall coming down the stairs. If that doesnt work you get another chance when fallen start spawning in the middle of the room. If that dont work, you get another chance when eris says "keep moving guardian" there will be fallen coming out of a bottle neck. For nova bomb use: shatter, bloom and annihalation for the best results