You probably won't have this trophy unlocked after completing the game, so use Chapter Select to load up Chapter 8 - The Citadel on Very Easy difficulty. Progress to the point where you have to use the binoculars to spot the North Star and the entrance below it. You'll then have to run from some guys shooting RPGs at you.
At the end of this path, you'll find a Dragon Sniper rifle and some ammo for it (The rifle will respawn as well). Now use the rifle to shoot 4 of the RPG enemies. DO NOT shoot the final enemy. Instead, reload the checkpoint and you'll begin again right there where the sniper rifle is, so pick it up and do it again until you get your 30 kills and your trophy.
Note: The trophy will unlock as soon as you get the 30th kill.