Once you kill death singer go to the tower on the right of Crota's pedestal and just snipe and blow up the knights on right and go in and out of side after to kill any knights by the crystal

Yeah, after you kill every knight in the building two sword knights spawn in the crystal room. When they spawn that means you dont have to kill anymore sword knights. I usually stand ontop if the door on right to kill the ones outside. What console are you on?