Hangman Trophy in Uncharted 3: Drake's Deception
Defeat 20 enemies with gunfire by aiming while hanging
How to unlock HangmanThis will unlock for accumulating 20 cumulative kills whilst hanging, which is done by firing on enemies whilst you are supported by anything other than the ground. The environment is full of climbable objects and indeed much of it is required to be traversed to move on with the game.
It's very likely you will earn this in two playthroughs, maybe even one if you make the most of the opportunities when they come up. The thing with this though is that it is Side-Arm weapons only that you can draw whilst hanging so try not to carry around a Pistole or a Micro too much when focusing on this as they aren't really reliable over a distance and hanging often means being at a distance. TAU Sniper and Mag 5 guns will be ideal for this trophy, but a trusty Para 9 will also suffice.
Chapter 12 - Abducted has a very expansive platforming section to get to the radio tower and also presents plenty of opportunities for kills whilst hanging, make the most of these and if you need to farm any kills then clean out the waves of enemies as you ascend the side of the huge freight liner and repeat as necessary.
The trophy will unlock when you kill your 20th enemy whilst hanging.