Apparently there actually is a way.
Just run Nightmares on the moon.
You can blast through them (850lvl) kill the champions and get either an umbral engram at the end or 50 of the resource to focus them.
There are youtubers out there doing it below 2min but my build is not that good, at least for now.
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Nice!! That's a great tip - I've run 9 of them this week, should have realised myself tbh!