Once you’ve beaten your opponent, there will be a short period of time where they just stand there swaying before they fall over and die. You have these few seconds to complete the combination of moves necessary to perform a fatality maneuver. Each character has their own unique set of fatalities and you’ll know you’ve performed it properly when you see the animation of your foe’s grisly demise.
This achievement must be accomplished in an Xbox Live game, but if you’re boosting your online wins, then you’ll have plenty of opportunities to get it. Here is a link to a list of all the moves for each character. Since you have to beat your opponent first, its best to use someone that you’re comfortable fighting with, but I found Kitana’s to be fairly straightforward:
Back, , Forward, Forward, .