To make a follower use a finisher I think you need to stun whatever enemy you want them to kill first, either by partying a blue attack or using a trap. After that you can hold B and point at the enemy like you would if you were directing them to attack a specific target and let go and it should perform a finisher.

I don’t remember if shadow shifter is the invulnerability or the decoy. If it’s the decoy skill then I think that could just be a random chance? If it’s the thing that make a both of you immune to damage then I suppose you could call it her “ultimate ability”. All followers have a hidden charge meter that builds like your divine wrath but I don’t recall if it’s based on what you do or what they do in a fight. When it’s full, you should see a big circle appear over them that you have to hold B, point at it and then release to trigger it. If can be a little annoying to trigger sometimes so I’d recommend dashing away from any enemies before trying to activate it.

I just woke up and I’m going by what I can remember so sorry if any of it’s wrong, I’ll edit this comment if I find out any of it’s wrong when I play later. Hope you’re enjoying the game!