In Mission 1, Chapter 2, after the soldier blows the passageway up with a C4, you will be ambushed by 2 grenadiers on the roof of the building. Wait for one of them to equip a grenade, then enter Gangsta Fire. Take aim with and wait for the grenade to take flight. Quickly fire and destroy the grenade before it gets too far from the grenadier. This achievement may take a few attempts.
In Mission 4, Chapter 2, the key is patience. Look for a rocket guy and eliminate all the other enemies. Then go into Gangsta Fire and aim at the rocket guy. Wait for him to fire, and then immediately fire. It will blow the rocket and kill the enemy. If it worked correctly, the achievement will pop immediately.