Grenades do not unlock until towards the end of Sequence 3. This achievement is most easily unlocked during free roam, as I have yet to find a mission that reliably groups enemies together.
A good location I found for this achievement was at Vallée Verte in the Fort La Croix region - specifically co-ordinates (720-715). There will be four red circles on the map, which represent patrolling enemies. Most had three people in the group, but I found one that contained five.
The groups follow specific routes, so follow them to see where they go. If you can find a place to hide, either in the trees or the undergrowth, then make use of it. If not, stand a fair distance away as entering the red circle will cause the enemies to attack you.
You want to shoot a firecracker dart at the ground in front of them. This will cause them to stop moving and investigate the noise. When they approach it, and are all gathered together, aim at them with the berserk grenade and fire. They should all be contained within the area-of-effect of the grenade, and the achievement will unlock.
This achievement can be easily unlocked as well. Simply change the ammo in the air rifle to sleep grenades, and fire on the group once again.
The video below shows the location mentioned, and roughly where you should aim for maximum effect.