Trying to get “Fatale” Calling card… What did i do wrong?

-Heard it was easier on citadel with the fire sword

-initially i used mutant injection to get their health as low as possible

-what i tried to in this clip here was go to the hallway and get them close as possible before using the special

My thoughts on why this didnt work is either bc i used mutant injection first and then tried to one shot them?

Or they have to die simultaneously. I dont know why but when i tried to use the special i got a parry instead and i think thats why it only killed the amalgam with the lowest health first (i was tryna get the one in front of me whos health was higher)

Nonetheless if you have any tips on how i can succeed on winning this CC it would be helpful