Buddy System Trophy in Uncharted 3: Drake's Deception
Complete one Cooperative Multiplayer game
How to unlock Buddy SystemAlthough this is categorised as an Online Trophy, it is actually possible to unlock this offline via Splitscreen mode but you would need a second controller. To do this, simply plug in another controller and choose 'CO-OP' from the main menu instead of Multiplayer and 'Splitscreen' instead of 'Play Online' to be taken to a lobby where you can select which type of mode to play.
However you choose to do this, Splitscreen or offline, the process is the same. Co-Op game modes are as follows:
Co-Op Hunter Arena
Co-Op Adventure (Fort is DLC only)
Co-Op Shade Survival (DLC only)
The good thing about this trophy is that you do not have to be successful in any of these game modes, you just have to have played them. The only exception is if you quit out or the other party members leave and you are left on your own. If either of these happen then you will need to try again but providing you either complete your objective or run out of lives trying, you will unlock the trophy.
All of these game modes are great fun especially with a friend online so if you are going for this trophy together with a friend, then I would strongly recommend doing a Co-Op Adventure map. The trophy will unlock either when the last life has been lost or the objective has been met.