This can be boosted very easily in exhibition match with a friend.
The easiest way to match with a friend is as follows.
Ensure you are not in a party.
Select exhibition match.
Select host match.
Select your friend to join the party it prompts you to make.
Get your pal to play possum.
The current character list is:
Update: A number of people have reported this as buggy and based on the comments and messages I have received here and Xbox Live there are 2 possible solutions as follows:
1 - Follow my solution but make sure you win all 6 fights consecutively in 1 sitting with a friend. (This is what I did before knowing it was glitchy and some people have reported this working for them).
2 - Do this on ranked Xbox Live (Which I do not really see as a solution at all given how horrible I am online, but people have told me it works so I thought I would include it).
Update 2 - Credit to KiDLUCiF3R for this info
It is believed the glitch occurs if accepting the game invite from the Xbox dash rather than the game main menu.
When tested, it seemed the person accepting the invite must be past the "checking for DLC" notice on the Killer Instinct main menu.
Solution for a fix is currently unknown. Normal things such as clearing cache or re-downloading the game are more difficult to achieve on Xbox One. A factory reset of the console might work but has associated risks. Or signing in to a different console.