Unlock demolition perk 7

Demolition perk 7 is Sentinel and is unlocked by getting 5 kills with thrown back grenades.

A great place to farm this trophy/perk is the same place as the alternate method location for Demolition perk 6 (see Vaporize). Except this time you will have to wait for the enemy to throw the grenade. When you see an enemy about to throw a grenade back up from the door and get into position, when he does throw it press [ Register or Signin to view external links. ] very quickly to throw it back. Make sure you are aiming high enough to throw the grenade far enough to kill them. Since there are multiple enemies close together in this room, you should easily get 3 or 4 kills in one try. Restart the checkpoint to farm more of these kills if needed. Refer to the following video if you need a visual aid: