Right when your on your way to get alex right befor the Endurance, you will be in a building that has 3 stories and you fight enemies the whole way up. You will know your in the right building if the first floor had a shield guy you had to kill.
Anyway on the second floor there will be a bunch of guys are shooting at you from the roof, they are in front of a red barrel, don't shoot it because lucky you, you have one of three guys up there with dynamite. Just wait for him to pull it out and shoot him with the assault rifle. He drops it, kills all three of them, and the barrel explodes for extra effect. Done. Achievement unlocked.