The Shrieker won’t open in the Catalyst mission. Any ideas?

If it never opens, that sounds more like the encounter hasn’t triggered than a combat issue. Double-check that the preceding objective is complete, then try leaving and starting the mission again. That may reset a stuck trigger, though it isn’t guaranteed.

I’d avoid spending ages trying to force it. Make sure you’ve interacted with anything the mission is asking you to, and if the Shrieker still doesn’t react, restart the activity. If it keeps happening at the same point, note exactly what you did beforehand.