Chapter 5 - Section "Make Him Pay"
A great place to do this is halfway through the fifth chapter. You will come to an area that looks like a little marketplace. Lots of enemies will attack at once, more than at any other point of the story. You really can't miss this, it's quite a big fight.
Quickly throw a grenade when you see the big group of enemies. You can cook the grenade by holding down R2. That way, they won't have any chance to take cover before the grenade explodes.