Occasionally Overseers (the masked guys) will throw grenades at you, which you can pick up with and throw back with like a traditional item you can pickup and throw. You may want Time Bend for this to make it a bit easier. The achievement can be a little glitchy, so try and kill the guy by getting a direct hit with the grenade, which seems to work 100% of the time.
In mission 2 is a great place to do this. After entering Holger Square you have to help a trapped guy named "High Overseer Martin" (cannot be missed). Right in front of him is a normal Overseer. Just make some noise and run around until he throws a grenade at you.