Finally I have discovered what was the inconsistency with Jaqcui Xray brutality...
the first hit of the move (the uppercut) must kill the opponent (so the opponent must have 5% health or less)... if you connect on an opponent with more than 5% of health bar, the xray will kill the opponent, but the brutality won't trigger, even if you hold "down" as the move connects... and its not necessary to combo the move... (but comboing is easier, because if your opponent have more than 5% health, probably the combo will drain health enough so the xray will connect with 5% or less remaining)
So the requirements are... 1- opponent with 5% health or less when xray connects, and 2- Hold "down" as the move connects until the brutality triggers...
now I gotta find the sonya xray brutality requirement that I'm missing... :/