Chapter 22 is a good level to farm headshots if you don't have enough after finishing the game once. There are lots of enemies in this chapter, so it shouldn't take very long to get the trophy.
STEPS
1. Start from chapter select on easy.
2. Open the Bonuses menu and equip the Dragon Sniper.
3. Farm headshots until you have the trophy.
TIPS
- Instead of killing the first enemy you see from the ledge, you can climb up for an extra headshot opportunity instead. You may have to move slightly to the left to pull yourself up onto solid ground, since he's standing above you.