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 Sonya and spam
, when "Finish Him" appears on the screen perform your fatality.
1st Fatality (Mid): Down, Forward, Back,
2nd Fatality (Close): Back, Forward, Down,
Video for Reference: