If you haven't unlocked this trophy by the end of the game (and you probably won't), then use Chapter Select to load up Chapter 12 - Abducted, on Very East difficulty. After the initial fight scene and a bit of platforming, you'll come to the watery area with all the boats and platforms. This is where you can work on RPG kills.
First swim to the boat on the left that has the sniper up on top of it. There is an RPG with 2 shot on the platform just below where the sniper is. After using it, jump back in the water and swim all the way to the right side of the area (opposite the boat you were just on) to find an RPG with 1 shot lying in the open on a floating platform. Use that and then run into the container underneath the second mounted turret to find a third RPG with 3 shots lying under here.
After using up all the RPGs, just reload the checkpoint and you'll be back at the beginning of the watery area. Rinse and repeat until you get your 30 kills and your trophy.
Note: The trophy will unlock as soon as you get the 30th kill.