I also had a great deal of trouble with that quest. It was ten times the irritation, because it locked me out of the rest of the game while I was figuring it out.
I ended up taking on Nira first, because I found it easiest. Mostly just peaking around corners and sniping. When my smokescreen was up, I cast it and stepped in. I seemed to be safe there and got in some damage. When my health got low, I ran until it was back up. I had my wife announce when my heal or smokescreen was up so I didn't have to watch and could focus on the action. (She's wonderful.)
I found Amar too hard, and didn't do him until I got my frame back.
Boreal was the one I did second. You have to save the smokescreen to stop her heals. You throw the grenade so the cloud crosses the edge of her scream. Then you can walk in through it, and get in a shot to stop her heal, and probably a couple more before she's active again. Hint: If she disappears, she probably teleported behind you.
God bless! Jeger.