Brawlhalla

Since the previous solution just stated the painfully obvious where all you have to do is double KO opponents (duh!) to get the achievement, I wanted to add a couple of tidbits of information to actually help achieve the achievement.

First: It says you must get the Double KO's in matchmaking. This may be false. My achievement unlocked while playing offline in the tournament (I've only achieved 3 double ko's in free-for-all matchmaking by my count, but way more since I started messing around against bots).

Second: I found characters with high dexterity and double weapons to be the easiest at knocking off multiple enemies at once. My personal preference was Cassidy using her pistols. (What I found to work for me)

Third: I noticed while playing free-for-all matchmaking a number of lobbies I would come into would have a split screener (who was idle) and every so often the 3rd player was idle as well. This allowed for very easy double and tripple XO's. Kind of a cheap way to boost but some might find it an option.

To achieve a double KO you’ll need to defeat 2 opponents within a couple seconds of each other. Do this 10 times and the achievement will pop. Achievement is cumulative and can be done over multiple matches. I got this in under 40 games.