Near the end of chapter 1 (Prison) you will come to an area with the first 2 commanders. Jump out of the vent, kill the first one and turn left. Kill all 3 enemies and 1 dog and pick up the grenade that's on a box just before the bridge. Make sure you trigger the alarm.
On the opposite side of the bridge will be a bunch of enemies camping by some boxes. Throw the grenade in between them and you should kill 3 of them. If you don't, you can reload the checkpoint and start from the vent again.