In the first horde encounter where fallen and hive attack, there's some dust that needs cleaned. There's a point once you kill all the first enemies and "the hive hunger for you light" thralls come out of the doors one at a time very slowly. You have to stand and wait while they painstakingly exit the doors, and you cannot progress until they all come out. Not sure why you made it that way or if it's a bug, but the way that it is really makes me not want to do the strike. And I can imagine others feel this way. Thanks for taking time to read these!