Mortal Kombat 11

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 + and + or , 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!

I found the easier way of doing this if you struggle to button mash is goto AI first to 3 wins on the online section. If you win the fight against a real person using an AI as also yourself sometimes it will use the second fatality without you having to unlock it. You have to win the fight and it will always use a fatality. Also an easier way of getting hearts. I did this with each character winning and unlocked the achievement if it uses a alternate fatality. Just keep trying until the character you want does the second fatality. Got almost all second fatalitys doing this method

In this video guide I will show you how to perform two different fatalities with Cassie Cage. For the first 10 characters I just used the klassic towers to get the achievement progress for I Want It All. Make sure to put it on very easy to make things quicker. Also pick the shortest tower.
Achievement progress also counts if AI do a fatality.

I Heart U (Mid) - Down, Down, Forward, B
#Girlpower (Far) - Back, Down, Down, Back, Y

In the options (press Select) set number of rounds to 1 and when selecting opponent set AI difficulty to 'Very Easy' (can also use a second controller if available).

Hit the enemy until their health bar is drained at which point 'FINISH HIM' will appear on screen. Position your character in the appropriate spot and input the following commands:

I Heart U (Mid) - Down, Down, Forward, B
#Girlpower (Far) - Back, Down, Down, Back, Y

Then when the points are tallied hit 'Retry' to restart and perform the second finishing move.