Mortal Kombat X

You need 1.929.949 XP to reach level 65.

This is a 2 controller method using Kustom Kombat with four modfiers.

Now it's time to set your Kombat Kard

Background:
Brutality Ending; 200 XP per Brutality performed
You get this Background for doing 250 Brutalities. Thanks to Premoz.

Icon:
MK IX; 100 XP per match win
I think this was unlocked from the beginning but I need confirmation here as well.
Edit: This comes from completing the MK I Premier tower that is still up at the time of writing this. Thanks to PointySpred for confirming.
You can use the MK 1-4 Icons (and other following propably as well) since they give the same bonus.
They came from completing the classic premier towers.
Credit to Chucklestyle for pointing this out in the comments.

Border:
Judan; 250XP per match win
This unlocks from reaching level 50 with the Lin Kui Faction.
Use the guides from the "No Loyalty" achievement to get this done and keep in mind that you lose everything from the Faction you're a member of when switching, including the Backgrounds, Borders and Icons.
I would suggest to do this after having leveled every Faction to the max.
Edit: Every faction has a border that gives 250 XP for reaching level 50 with it. Thanks again PointySpred.


Set up a match und choose Mileena because in my opinion her Brutality is one of the fastest, the second player choice doesn't matter.
You can choose Shinnok and do his back, forward, (hold) X Brutality which is at least as fast as Mileena's.
Credit goes to TNelson1995

Edit: Its Susi stated in the comments that he was using Kenshi to boost this by himself. He remapped the throw button (RB) to "A" in the options doing only one round before restarting. Kenshi's Brutality is his normal throw. Great idea btw!
A win gives him 1660 points. This is much easier of course but my method nets you more than 500 more XP for only a few seconds more. So it's your decision how to do it.

Latest edit 6th of June 2020:
Credit goes to AziK HaYaLeT

Well Tanya gives you 250 XP for Background instead of Brutality 200 XP (you need to win 100 matches with her which is easy you will need to win 1k matches here.) That 50 XP can save you some time and for brutality it is so quick with LB only (press UP while throwing). It takes about 20 seconds and 1720 XP.

If you go with method of this guide it will give 500 XP more but you will lose at least 6-7 seconds. 1720 XP for 20 seconds or 2220 XP for 27 seconds. Math is easy 1720 XP is just better based on XP per seconds.

back to my original solution

Player 1 :
Double Damage
Powerful Jump Attacks

Player 2 modifiers:
Brutality Kombat
Killer Jumps ( set to "opponent")


Since you put Killer Jumps in your own favor you'll only need two attacks to win the first round.

When the match starts do a forward jump front punch followed by a forward jump front kick to end the first round within 1 second.
when the second round starts use mileenas rolling ball brutality by putting in back, down (and keep holding it), B.
This ends the second round within 5 seconds including watching the victory animation.
You will have earned 2.226 XP.
Retry the match with the same modifiers and rinse and repeat.
Good luck!

If you give negative votes please state why in the comments so I can improve my solution.

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!

I looked at quite a handful of different methods considered to be the fastest, however none of which seem to be as fast as the one I found. You will need a certain Kustom Kombat Modifiers, otherwise you won't get nearly the same amount of XP per match. There are 3 optional Modifiers that I recommend to speed the process up (one of which I didn't have). I strongly encourage using the same Gameplay settings and the same Kombat Kards. (If you do not have the same Kombat Kards, you need to unlock them from challenges, or you can select which ones will give you the most XP. If you do not have the Infinite Super Meter Kustom Kombat Modifier, you will need to unlock it through the Living Towers. The recommended Modifiers would help but aren't required. You also need 2 controllers for this method.)

The goal is to make the match as long as possible, by using modifiers like Weakened (if it is unlocked), giving your opponent Health Enchantment and Health Regen (if it is unlocked), and disabling the timer and having each match last for 5 rounds. You will need the Infinite Super Meter modifier to have an unlimited use of X-Ray attacks. Equipping the Kombat Kards rewarding you with the most XP possible is strongly encouraged. Sub-Zero is the recommended character because activating his X-Ray doesn't require to block an enemy (Sonya Blade) or dash into the enemy (Scorpion).

1. Go into Gameplay settings to disable the timer, and I strongly recommend you set the match to 5 rounds, since the time transitioning into rounds is faster than the time transitioning into matches.

2. I strongly recommend you use the same Kombat Kards that I had on: (you will need to have these unlocked, otherwise, use which ones will give you the most XP)
Background: Clean Break - +200 XP per X-Ray performed
Icon: Lin Kuei Leader - +50 XP per X-Ray performed
Border: Judan - +250 XP per Match Win

3. Two Player --> Kustom Kombat --> Have both players be Sub-Zero (Cryomancer variant)
*Make sure you have the required modifier and use the recommended modifiers if unlocked as well*

Player 1: Modifier 1 - Infinite Super Meter (Required); Modifier 2 - Weakened (Recommended)

Player 2: Modifier 1 - Health Enchantment (Recommended); Modifier 2 - Health Regen (Recommended)

I personally had Infinite Super Meter and Weakened for myself, and my second controller had Health Enchantment. With those Kustom Kombat Modifiers, the Gameplay settings and Kombat Kards, I was able to get a minimum of 19475 XP per match. No matter what you try, this achievement will be a grind, but this is the fastest way I've ever seen, so definitely give it a shot if you have the right modifiers.

Just want to say that if you're going to use 2-controller method (don't forget about 200 XP per X-Ray Background) and if you have time to kill - you can customize your match-up with Weakened, Infinite Gauge and Two Enhanced Health Modifiers. By using them - you'd be able to grind 28 700 XP per match