Receive 5 medals.

I suggest working on this online trophy first, because along the way you will pick up a few other online trophies as well. For this particular trophy, you need to earn five medals. Press Triangle Button at the main menu to go to your pilot's profile. From there, press R1 Button until you reach the medals tab. Here you will see a complete overview of all the medals that can be earned and the requirements for them.

The easiest five medals are:

War Medal 1939-1945: Win 5 Versus matches using British planes.
France and Germany Star: Win 10 Versus matches using British planes on any Ruhr map.
Good Conduct Medal: Win 5 Versus matches using US planes.
Asiatic-Pacific Campaign Medal: Win 10 Versus matches using US planes.
Order of Military Merit: Win 10 Versus matches using Japanese planes.