To get this really fast press
on the main menu and change "Rounds to win" from 2 to 1. Then go to local and play against a second controller or the CPU, if you're fighting the CPU I'd highly suggest setting it to "Very Easy", making all the achievements far easier! Select Cassie Cage and spam
, when "Finish Him" appears on the screen perform your fatality.
1st Fatality (Mid): Down, Down, Forward,
2nd Fatality (Far): Back, Down, Down, Back,
Video for Reference:
Note: for the second fatality the Distante is Far even though the game says Mid, some fatalities have the wrong text, thanks to SOUL OF GAME24 for pointing it out!