Very easy to do, all you need is a 2nd player in the game with you.
If you don't have anyone to play with and you don't want to try this in public matches, simply sign a second controller into a guest account.
Walk up to your teammate and melee them by clicking down on
They must then accept the duel by returning your melee with one of their own.
You are now in a duel, where anything goes! Do whatever you can to kill your teammate. When one of you goes into Fight for you Life the duel is over.
To earn this trophy you must be the one to defeat your opponent.