In order to get the trophy that's called "Cooking With Gas", you will have to shoot down an enemy rocket projectile while it is in mid-air.
In some of the later story missions, you'll come across rooms in which you may have to sneak by guards without starting a fight. There will be a ton of guards patrolling those kinds of areas and one of them will be a man dressed in a red suit who is equipped with a rocket launcher. Get into a cover position and draw them into a fight. During the battle, that red guard will aim his rocket launcher and fire. As it is flying towards your cover position, make sure that your yellow target reticle is lined up correctly with the rocket by moving around the [cn_rs] and then pressing [cn_r2] to fire. If done correctly, your bullet will hit the rocket and it will explode in mid-air, thus awarding you the trophy.
Have fun and good luck!