You have to kill 5 enemies with thrown back grenades. This is hard to do, as the enemies suck at throwing them, they have short timers, and you cant hold it to aim and throw back really. My method made it very easy though.
You can do this with your own grenades. I turned the difficulty down all the way, grabbed extra health and armor, then would throw a grenade at my feet when near an enemy. Then pick it up and point in the direction of the enemy.
This can be done pretty much anywhere. I did on chapter 6, as there is a lot of health, armor and grenade pickups, and lots of enemies not far in to the level. Its also where I was in the game when I worked my way to this perk. Just reload checkpoint when you are out of grenades or if you die, just keep doing it until you get 5.