This page contains a detailed walkthrough for a silver trophy named "Right Under Your Nose", a reward for destroying the helicopter without defeating any other opponent.
The opportunity to score this trophy appears in the final part of the eight chapter of the game (Partners), during your attempt to retrieve the Tusk from the hands of Shoreline mercenaries.
One of the pre-requisites of the trophy sees you focus on destroying the helicopter only, leaving out all remaining opponents. Consider changing the difficulty level to one of the lesser ones before you begin the section.
You are going to need two rockets. The rocket launchers can be found in the orange weapon crates located in different parts of the trainyard (5 in total). Jump into the water and make your way to the central part of the area. Continue by moving through the high grass to a wagon placed underneath the bridge. Enter the wagon (watch out for a nearby sniper) and open the chest shown on the screenshot above.
The second accessible chest is located at the other end of the trainyard, next to a large wall and a locked gate (screenshot above).
Note - If you've made it to the end of the game earlier, you can pick up a single launcher and utilize an infinite ammo cheat (activated in the game menu).
If you've found at least two rockets, find a location that will be easy to guard against incoming opponents (firing at the chopper will raise an alarm) and fire the rocket when the helicopter appears next to your position. Fire the second one soon afterwards, as you may miss or be subjected to enemy fire (remember that you cannot touch other opponents beside the chopper)
If you've managed to hit with both rockets, search for an opportunity to employ your grappling hook. The action may require climbing to one of the wagons. Catch up to the helicopter and start climbing, as you'll be engaged by other mercenaries found in the trainyard.
Continue climbing the rope until you've made it to the side door of the helicopter. Unlock them to begin an encounter with Orca, an elite Shoreline mercenary. The interactive scene will see you press Square and Triangle in rapid succession. The ensuing melee will result in the death of the pilot, initiating a cut-scene that will unlock the trophy.