Shortly after defeating The Pain, you'll have to wade through a river in the area known as Ponizovje South. After you get to the end of the area, past the drones, don't go through the opening straight ahead. Instead, go to the opening to the left to the area called Ponizovje West. Here, knock out or kill the guards and go to the little room that was behind the further guard. In here is some TNT and a sniper. Pick up the TNT, equip it, press
to lay it down, leave the room and get out of range of the explosion, and then press
to detonate the TNT. And there's your trophy.