You have to keep using rifle attacks until your soul gem (on the upper right hand corner) glows. There’s also a voice line indication that she’s ready to use the attack. Then just do D, DF, F, a+b ( down front a+b) to activate.
A bonus is when you win (I think with a Fatal Tiro) and lose once and it’s round three. Try and use the attack again then...
well that’s for you to find out (or someone else can tell you, unless you want to be surprised)