You need to kill 3 or more enemies with a single grenade
Later on in Chapter 1 you will come to your first hostage situation, you will come to a cut-scene were they explain “Civilians” and how to rescue them. Just have your co-op AI or real partner go up to the first guy and take him hostage. Then Tie Down the other 2 guards. Now they will be laying in a circle on the floor, toss a grenade in the middle. You will probably kill the hostages so just restart the checkpoint.
You can also use a grenade launcher to help get this later on in the game. You can check what your highest grenade kill is by going into the start menu, going to Statistics and looking for "Most Kills With A Single Grenade".
A good spot for this is Chapter 3 just before you enter the zoo after the guy on the outside TV monitor talks to you. As the ambush begins a ton of guys swarm over a wall, just throw and reload checkpoint if you miss.