To start, I understand you gain bonus progress in Heresy activities, but I just did a GM nightfall which had about 10 Banes spawn. Of which I killed at least 6, but I gained only 3% progress.
Given banes only spawn in harder content, and only about 10-12 spawn per run, your in for a hefty grind. Even of the bonus progress is 2x, that's still at least 10 master runs of the nether or court. That's not even considering if you fail, or if it even counts teammate kills as progress for you.
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At this point, Bungie needs to either make Seasonal Challenges retroactive, or stop overestimating how much time we're willing to put into activities. We're thirteen weeks into the Season, nobody feels like getting 100-200 Bane kills at this time. Especially since we've already ran those activities plenty of times for other reasons, we're sick of them. Calling it now, instead of adjusting the requirement, Bungie is just going to auto complete it. Which if I'm mistaken, would make this the third challenge this Season Bungie auto completed.