Go to Kortz Plaza. Traverse on foot from the lot to the upper deck behind the complex.
Head to the main plaza, hiding behind cover and hitting down FIB, IAA, and Merryweather members. Move cautiously so you can ambush some enemies from behind.
Start sniping the baddies at the courtyard using Trevor, then swap back to Michael. Flush down the chopper with a rocket, and inch your way to the plaza.
Keep switching between Trevor and Michael if needed, and rush to the parking lot. Shoot down the pursuing helicopter and go to the city.