I can confirm that sniping the deathsingers from underneath the platform where you spawn grants the two grimoire cards you don't get from the raid.
Patience is necessary (you are teleported if you fire shots too close together), and a high impact sniper is recommended.
Having more people speeds up process but it's easily soloable.
[spoiler]check my grimoire for proof, enemies-exalted hive[/spoiler]
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Known forever, just takes so long that no one does it