Go through 20 gun drops in one-armed combat in "Captured."

Mission 14: Captured

After you break your arm, and Gideon helps you escape, you will only be able to use one hand to fire your weapons. This means you cannot reload. For this trophy, you need to pick up 20 guns from the floor. You don't need to fire any bullets, just as long as you pick up 20 in the mission. There is about 30 guns you can pick up all together. Most of them will come from dead enemies, but there are also some on the wall in a couple of rooms. As long as you pick up every gun you see, then you should have no problem getting 20 during the mission. The trophy will unlock once you pick up the 20th gun.