Well, for me, as long as i'm facing the opponent and hit the buttons correctly then it works.

when it says FINISH HIM change the fighting stance to FATALITY then do the button combination and thats that


NOTE:you have to do it quickly

well u dont actually need to be in the fatallity stance but to do that just chance stances normally and you will see on saying fatallity. u need to press the buttons quite fast but there is no need to press very fast.