This is extremely easy. Just start a game with as many characters as possible (8 if you have one controller, 16 if you have two).
When the match starts, press all the buttons you can to drop as many players as possible at once. Now, press all the buttons again to deploy each character's pulse attack. The idea is that one character needs to kill two other players. With a board overcrowded with players, this should unlock almost instantly as soon as you follow these instructions.
If it doesn't work the first time, start a new match, drop all the players and only use a few buttons again to attack.