This can be done in local versus with 2 controllers.
Obviously you're going to play as Harley Quinn and pick Joker as you opponent.
This is one of Harley's combo moves.
It's done by hitting
You want to do it with your opponent at around 15 percent health. Less than that and you'll defeat your opponent before she completes the combo. If done correctly Harley will knock the Joker down to the ground and smash his head with a baseball bat a few times.
As pointed out in the comments you can use
in place of
. Easier than trying to hitting 2 buttons simultaneously.