After the rocket to the snipers face, while they're still on the roof. You had to cover your guys down on the street until they got across. I got it a few seconds after they cleared and the enemy started storming the building you're on top of.
It didn't pop for me at frist either, after fending off the 3 waves of people "DOC" will say thnaks and the three will take off. then stick around and shoot some of the goons running to the bottom of the building while your supposed to be leaving the roof
top. that's where it popped for mo on the third try