In order to get this trophy you need to win 10 battles in the Colosseum (the game's PvP arena). The Colosseum is unlocked for one of your characters after beating the game on normal difficulty. It is located in the gate area, with an entrance in the gardens that are before the portal.
If you do not want to go for this trophy by playing online you can get it locally with only one controller by playing against AI opponents. In the Dragon Haven Inn add one AI companion to your party and then go to the Colosseum. Make sure network settings are set to Offline. It will bring up a list of battle options - you want to make sure that allow AI allies is set, the others can be set to anything but it will be fastest if you set LP to zero. Once you enter the Colosseum it will display a message that it is searching for other players, but within a few seconds (if you are set to offline) it will start the match with you against your AI companion. Once you defeat them, select to continue the match and it will queue up the same fight for you. Do this ten times and the trophy is yours.
Although you shouldn't have any troubles defeating the AI players anyways, you can make this even more of a slam dunk by bringing a very low level ally into the match versus you (there seems to be no level handicapping going on). If you do not have a low level ally currently revived at your Inn, simply start a new character (using the same save file) and play the first mission, making sure to save the bones halfway through the level (you will need to play through with each of the characters eventually anyways for the character specific game completion trophies). Revive the (likely lvl 1) adventurer found in this way and then switch back to your character who has beaten the game and add them to your party (or you can revive them with main character, the list of allies and the list of bones are shared among all characters on a single save file).