To get this trophy you have to play the multiplayer to level 15.
The problem is that this takes a while and is very boring. To level up all you have to do is after picking your faction (I recommend the assassin's because a majority of the players are picking that though it doesn't matter at all.) Now you will either do one of the two options attacking or defending. To attack you will click on the crossing swords icon to attack making you pick a person to attack and a person you choose. and there's defending which is you defending one of your own bases until its lost or the team wins.
The main problem is that the game is boring and for a 10 hour grind of just picking attacks then actually doing them, is kind of boring and will make it feel like for ever, however defending allows xp to be earned while idling, and you earn double xp fighting near or within your home location.
Have fun, or try to.