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 take out the first couple of enemies, then use the OWL to stun a big group. Walk up to them and press to activate the adrenaline. Adrenaline can be picked up at the checkpoint before this fight. It will regenerate your health immediately and slow down time. To slow down time you need to aim using L2. Try to get really close to the enemies, so it's easier to get headshots. Your standard weapon is fine to do this.