For those like me who don't have two controllers, here's a one controller solution that quickly helped me to get the achievement (based off some information from
Dr Lipschitz's solution for the "No Loyalty" achievement so many thanks to him!):
First thing's first; only do this when it's one of the very last things you need to do to save grinding time.
Ok, now set all your Kombat Kard bonuses to anything with high XP payouts. If you're level 50 in your current faction, you should have a 250XP border to use. I personally used the MK IX icon for 100XP and then the "Brutal Endng" background for 200XP per Brutality performed. Just pick whichever your highest XP bonuses are for each category.
Now, make sure matches are set to
ONE round and head into a "Test Your Luck" fight. Pick Shinnok (variation doesn't matter), set the difficulty to very easy and set 3-5 modifiers. As soon as the match starts, you want to be looking out for the
Brutality Kombat modifier. If you don't see it, pause and restart until you see it. It may take a few minutes until it shows, but it will eventually. When you do get it, you want to make sure that there are no health degenerating ones as well (such as Bleeding or Bad Special) as these will cause you to get less XP once the match ends. While not essential, it just helps to speed up the process.
The second you begin fighting, perform Shinnok's "Gory Hole" brutality (
and
HOLD DOWN 
). The reason we've picked Shinnok is because he has one of the fastest brutalities in the game and you won't need to touch the opponent like many of the other characters.
If successful, the match will end within one second. I was gaining 1,600XP every match with this method and managed to get all the way from level 63-64 in half an hour. To go again, pick the
SECOND OPTION DOWN to have a rematch with the same modifiers, then simply repeat the process again.
On the odd occasion, the AI will block/swerve out of the way and hit you so you will only gain 1.3k-1.4k XP but 99% of the time you'll easily be getting a flawless victory. To ease the boredom, take a few breaks with towers (I kept going back to the hourly towers for Kustom Kombat Modifiers inbetween) or listen to some music or whatever. Once you find a rhythm, it'll go by very quickly. 20 seconds for 1,600XP isn't bad!