"Crota's soul may be defeated, but his armies remain. See to their extermination." - Eris
"Crota's soul sleeps again, but his disciples still fight in his name."
"You don't fear the shadows. Maybe there is hope for the Light."
It helps to run the Strike: The Summoning Pits. Just go through this strike once should yield all the needed kills.
You can also start and run Siege of the Warmind. Completing that (or struggling through it) will get you most to all the kills in a run or two.