If you haven't managed to unlock this trophy by the time you complete the game, never fear, there is a quick and easy way to finish this off.
First, from the main menu go to Bonuses/Store/Weapons and purchase the FAL Assault Rifle. Now go to Bonuses/Store/Tweaks and purchase Infinite Ammo.
Use Chapter Select to load up Chapter 4 - The Dig, and set the difficulty to Very Easy. When the Chapter starts, hit pause and use the menus to select the FAL and "Give Weapon", the use them again to "Enable" Infinite Ammo. Make your way a short ways into the chapter until you get to the shed where you receive the grenade tutorial. After the cutscene and the tutorial, the bad guys will start coming. Equip your FAL and press L1 to aim, then click RS to bring up the scope. All that's left now is to start headshotting away. Around 20 enemies will spawn in this area, so go to town on them. When Sully get surrounded up on the high ground, hit pause and Reload Checkpoint to start back at the beginning. You will have to go into the menus to re-equip your FAL each time, but meh.
Rinse and repeat until you get your 250 headshots and your trophy.
Note: The trophy will unlock as soon as you get the 250th headshot.