This can be done at many times during the campaign. I suggest refraining from using grenades on the stealth missions, but just spam with grenades on any of the assault missions and you're sure to unlock this.
If you want an exact spot, however, your first opportunity is during the first mission where you are told to hide in the bushes because there are enemies coming. When you spot them, chuck a grenade in between them (with right bumper) and *boom*. Achievement unlocked.