There is 2 ways in which you can do this trophy:
Option 1 (super easy) have a second controller, and start up a splitscreen game. Once the game starts, melee eachother on the controller and let the person win that wants the trophy. This will also gain you (if done right) the win without taking damage trophy.
Option 2: Go online and melee and of the people that you are playing with. When they challenge the duel, simply just beat them at it. Easy as that!
If you vote negative, please state why so I can improve my (future) guides.