Mortal Kombat 1

*Updated to include the fantastic solution by dante2236 and Kaiologyy*

There are couple of ways to speed this up but this and the 5 masteries achievement remain a significant grind and what you will spend your majority of time with the game doing.

Before working on this achievement, complete Invasion mode. This will get you a bunch of achievements, as well as most of the XP needed for at least one Kameo mastery, depending on how much you explore every map versus running straight for the boss. Your invasion level should also be at or above level 20.

Active Method
This is quicker but requires more active attention.

Step 1: Travel to the shop in Feijing village and go right once to find the match Triple Threat (You can use any Endurance match but I like this one because of how close it is).

Step 2: Fight the match and finish with a Brutality. If you trigger"Finish Them," finish with a fatality.

If you're at Invasion level 20 or above, each of your opponents should die in one uppercut, but this could vary depending on your character's element. You can respec and put all your points into Attack if you want to ensure it's one uppercut only.

If you can get a flawless victory, this doubles your victory XP for the match. This is reasonably easy to achieve on the characters weakest against you at this level but can sometimes be messed up depending on what the AI decides to do.

If you're struggling, you can make it easier by using a magic armour consumable or an amulet that gives you magic armour. (This isn't guaranteed as they can attack you while you're activating the amulet, and consumables need to purchased and aren't always for sale).

Step 3: Repeat until your Kameo character hits level 15, which will unlock the achievement.

Passive Method
This is a lot slower but requires little input from you, allowing you to just tap the A button from time to time while you're doing something else. If you have a turbo controller this method can be fully idled.

Step 1: Find a Survival match on an invasion board. There's usually at least 1-2 of these on each invasion board, and require you to dodge obstacles for a period of time.

Step 2: Start the match. You can leave your character alone, at this level even if you're hit by every single projectile you'll likely still be nearly full health. The match will end and you will receive a small amount of XP.

Step 3: Repeat. On a turbo controller, leave A on, otherwise just keep tapping A for the match and XP to repeat themselves.

The above methods will also work for the 5 character mastery.