"To begin this mission go to the pointed place and meet with the gang. A moment later the encounter with the group of thugs begins. It's a long and tough battle so be careful. Retaliate all attacks and use the environment.
After defeating the second wave of enemies, your companion calls you on the top of the building. You get a grenade launcher with which you have to eliminate next gangsters. There'll be many of them so wait until they approach the cars and then blow up the vehicles. You'll spare an ammo in that way.
When the last enemy dies, enter the free car and drive to the safe place.