and here a link to someone who really did a good job: https://docs.google.com/document/d/1ENKGPkqFztt83iNmKQNmPNyMyjcd3aXjF6xnFQD2BKA/edit
this console command guide is excellent
and should you find yourself in the same awkward position as I did and these damned chosen just won't spawn in their fortress - here is a post by rickswan to remedy the malady from the steam forums:
[i]Finally, I found something that worked for me, and here is how to do it:
enable the console. To do this, go to XCOM 2's properties, set launch options, and type "-allowconsole" (bonus: I also have "-nostartupmovies" but that's just to save time).
when you are stuck because you can't attack the sarcophagus and the Chosen Assassin won't spawn, press the console key (~) to bring up the console.
type "dropunit chosenassassin 1 1" - I believe I read that the first 1 makes her an enemy and the second 1 makes her spot you. This will spawn the Chosen Assassin (I didn't find the enemy name in a list anywhere, I just luckily typed the right name after a few guesses).
if the sarcophagus itself cannot be targeted (this was the case for me), you will have to attack it with explosives to do damage. For this, I had to use the "toggleunlimitedammo" console command so I would have enough grenades to destroy it. After a few grenades the plot progressed and the icon to attack it normally appeared, so I again entered "toggleunlimitedammo" to turn it off.
And that's it! Forcing her to spawn allowed the plot to progress and toggling unlimited ammo allowed me to damage the sarcophagus when there was no hit indicator (note that I don't think you can attack the sarcophagus until after you kill the Assassin once). After doing those things I was able to complete the mission. Other console commands you may find useful are "restartlevel" and "restartmap". Of course, I recommend turning off the console after getting through the mission so you don't tempt yourself ;)
Last edited by rickswan; 11 Feb, 2018 @ 2:15am [/i]