There is a very easy way to earn this trophy in chapter 7 (the on-rails chase sequence with the jeep). No setup required, and you can get it right at the beginning.
STEPS
1. Start from chapter select on easy.
2. Point the crosshairs all the way to the right and keep them there.
3. A few seconds into the chapter, 3 pirates will appear on the right, with an explosive barrel close to them. Shoot near the barrel with L2 (grenade launcher) to kill all 3 pirates in 1 explosion.
4. Restart from checkpoint, then repeat steps 2 and 3 four more times.
TIPS
- If you're having trouble, try pressing L2 when the crosshairs are slightly to the left of the red barrel.