I have found one of the easiest ways that requires minimal effort to achieve.
Go to Main menu, go to options, gameplay, rounds 1 (time doesn't matter)
Now back out and go to one player, single fight and choose Johnny Cage (I choose A-List for the variation) and for a laugh I chose Cassie Cage (any character will work)
Put on very easy, once in match make sure you perform his move Eye Popper with
at least 3 times in the match, when char is at low health 10-15% use Eye popper again (for the fourth time)
and he will hit the head off for a Brutality.
This is quick and easy and just retry on when match ends
Also even easier method if you have two controllers is to go to two player, Kustom Modifier and choose Brutality combat.
With this on just use
and it will end the match with a brutality, retry and rinse and repeat