I managed to kill one of the Ir Yuts and then the invisibility wore off, but then came back. tried to kill the second but got detected. managed to kill the first a few times, but couldn't manage to kill the second without being detected. I think there's a way to kill both without being detected and then steal the essence. The ultras may come into play at the end along with the adds, but haven't gotten that far.
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I killed all 6 of the deathsingers before stealing the essence but I only killed one of the ultras before being teleported. Killing everything like the Black Spindle mission is a good idea. You can fairly easily kill the two ir yuts if you stand near the crystal room but out of sight of the rest and shoot them with there back turned, but if you miss they sense your light, then the rest of them from the pool, after stealing the essence a full fireteam of stormcallers with transcendence should be able to clear the adds and if cleared fast enough hopefully the ultras as well. Something weird happened when I was trying in the story mission, after killing the two ir yuts I killed one of the other ir sisters pretty quickly(considering) and she didn't scream, it could of just been a glitch but it might be something, also I might of used the sleeper for the killing blow so maybe it's firing sound cancelled/bugged her scream.