This trophy is titled "Praetorian Guardsman" and it's for reaching level 30 in online versus modes. While you can rank up in both player and ranked matches, it's strongly suggested you rank yourself up in ranked as the points are exponentially better. Below is a means of getting quick ranked matches completed.
In order to get through matches quickly with a partner, it's suggested the player losing plays as Carl and the player winning plays as Haku-men. Carl Barrier Bursts (X, Square, Triangle and Circle at the same time) and then Haku-men uses his forward + C (O on default controls) while Carl moves in to the attack. This can make each round roughly 5-6 seconds at most.
If you rotate between the wins, you'll level up together and therefore give each other more exp. It'd be unwise to try ranking one person up THEN the other compared to simply trading off wins. If you have a friend with the trophy already, naturally it'll be pretty quick however you need to play 200 games regardless. This should unlock around your 150th match or so if you trade off wins, leaving plenty of room for error.