Killed three or more enemies by shooting a grenade
This can be done in Act 1 mission 2. You will get to a point where you will have to use stealth to get past the enemies after walking through a cave to reach the observation tower. Shoot the sniper patrolling the roof top so you can sneak past the enemies. Crouch your way past the enemies. You will then reach a point where you will shoot am enemy to cause a distraction. This will make three enemies run over to investigate. One of the soldier will be carrying a grenade; shoot the grenade when told and it will take out the three enemies around him.