This mission is given to you by the FIB ("B" on the map) and is available to Michael. It starts at Kortz Center and once you arrive there, go meet up with Dave. After a cutscene, you will be in the middle of a firefight between the FIB, Merryweather and the IAA. Now, at the beginning, you can run past the initial set of enemies or stay behind and kill them all.
The path ahead is blocked by a crashing chopper, so go to the right where your progress will be impeded by an active helicopter. The game will now switch to Trevor, so shoot the pilot and the chopper will crash. While still as Trevor, kill the enemies in the courtyard and then switch back to Michael.
You want to fight your way to Dave's position and you'll encounter plenty of enemies along the way, including a helicopter or two. Note that there's a health pack near one of the pillars before crossing the walkway leading to the stairs, should you need it.
Once you meet up with Dave again, keep killing enemies until you can switch to Trevor, who has some enemies on a balcony nearby. Kill them and the incoming helicopter, then go back to Michael. Fight your way, along with Dave, to the parking lot, where you will find plenty of more enemies.
Kill them all and grab a vehicle, but as you're leaving the area, another helicopter will appear. Note that this one will fire rockets and even the minigun at you, so as soon as you see it, get out and either take out the pilot or blow it up. Once you do, drive to Morningwood to meet up with Trevor and finish the mission.