You should of landed as heli on top of your pad. When they're all gathered around throw an airstrike flare.
I do usually do this when I'm in the city. I will get in my chopper and fly to where I see a lot of white dots and land on a building. I then call in the airstrike and throw the flare off the edge and then sit back and watch.
One of my funniest experiences was yesterday I kept killing these 2 guys, and they were like they are going to get their tanks. So I call Lester, go off radar and follow them at a distance until they get to their tanks. Before they can get out of their car, I throw a sticky on the back of the car, blow them up, and jump in one of their tanks. I quickly blow the other tank up and proceed to blow them up for the next 5 mins.