There are 7 opponents that you must do this to while playing as Batman.
Catwoman
Harley Quinn
Joker
Deadshot
Poison Ivy
Scarecrow
Bane
You must select the Arkham Asylum map and only need to do one level transition against each villian. The level transition is all the way to the right if you start on the default area. Force you opponent all the way there and hit
&
.
This can be done in local versus with 2 controllers.