Run-and-Gunner Trophy in Uncharted 3: Drake's Deception

Depending on how often you like to engage enemies up close then this could well come quite naturally. To kill an enemy in the open without aiming or being in cover and press to fire.

Attempting this during your Crushing playthrough won't end well more often than not so work on this on your first playthrough and take advantage of being able to absorb more damage. Be mindful though about what gun you attempt this with, because the ones with a spray cartridge like any Shotgun or the Pistole are the best guns to use, followed by powerful handguns like the Mag 5. Also try to take into account your enemy and their position as well as any armour they may be wearing, the heavy armour guys need to have their helmets blasted off then downed with a headshot so don't try any of these kills on those guys or they will kill you.

If by chance you complete 2 playthroughs and need a sweep up for this trophy, start Chapter 16 - One Shot at This and progress up to where you sneak into the second warehouse. Kill the guards close by and pick up one of the many SAS - 12 Shotguns lying around. There are heavy armour guys that will spawn but also plenty of normal enemies, easily a dozen in this area alone and if you can kill a heavy armour dude and grab his shotgun you will have the best gun in the game for this trophy. It won't take long to farm any remaining kills you need and as the kills are cumulative you wont need to worry about breaking any chain of kills.

Once you kill the 20th enemy this way, the trophy will unlock.