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 Skarlet and spam + and + or , when "Finish Him" appears on the screen perform your fatality.

1st Fatality (Mid): Forward, Back, Down,
2nd Fatality (Mid): Down, Down, Forward,

Video for Reference:

In this video guide I will show you how to perform two different fatalities with Skarlet. 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.
After getting your I Want It All achievement the method with be a lot faster. Once in the main menus press the start button to go into the options menu. Scroll down to change the rounds to win to one. Then back out, go into the fight menu, select local, make sure to press X to turn on the CPU, and then select your character. You only have the one health bar to deal with to get to the fatality.
Achievement progress also counts if AI do a fatality.

Bloody Mess (Mid) - Forward, Back, Down, A
Heart Condition (Mid) - Down, Down, Forward, B

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:

Bloody Mess (Mid) - Forward, Back, Down, A
Heart Condition (Mid) - Down, Down, Forward, B

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