Hey Guys!
So what you have to do is Defeat another Hero while playing as any Hero on Heroes vs Villains. I managed to get this done on my frist try on this game mode. I had two chances to pick a hero of my choice and on my second pick I was able to unlock. I found it easier to use Darth Vader but that's just me.
Here is my video gameplay for an idea on how to unlock this trophy.