You unlock the "Sentinel" perk and matching trophy for throwing back grenades and killing an enemy 5 times. To unlock the ability to throw back grenades, you must first have killed at least two enemies with a single grenade. That's pretty easy to do.
I got this trophy early in Chapter 13, after retrieving the luggage, cutting a hole into the control room, and pushing the button. There are three enemies in the main room. Take out the big one first, but leave the two normal ones alive. Now, move towards the door leading out to the big area. The two enemies will take cover behind things out in the main area. Just stand back from the door, and maybe over near the door to the control room, peeking or stepping out into view periodically. You'll hear one say "grenade" in German. When this happens, move back from the door but stand centered, so that as soon as the grenade is thrown, you can "catch" and throw it back with the [_] (square) button. Once you've killed one or both enemies, simply restart the checkpoint and repeat until you get five throw-back grenade kills.