You unlock Grenades during the last Missions of Sequence 3, after that, you can unlock the Achievement.
Fast Travel to the Viewpoint (659,-243) in the New York District East Villiage (North of the NY Map)
Stay on the Viewpoint and look to the left Side, you see some Enemies inspect some dead Bodies. Use the Firecracker to "gather" all Enemies at one Place, then choose the Sleep Grenade and shoot to the Enemies below. If you want you can also unlock the Instant Vikings Achievement here (The Video shows both). Choose after the Sleep Grenade the Berserk Grenade and shoot again.