Four

Head to the entrance of Fort Zancudo. As soon as you arrive, start shooting the first few soldiers you see. Don't get too comfortable though, because you'll need to turn around quickly.

Take cover behind the long wall that runs along the area. This wall is crucial for survival as you'll be facing an onslaught of vehicles and, most importantly, 3 tanks that fire their cannons regularly. These tanks can take you out in a single shot, so staying behind cover is key.

When a tank appears, use your grenade launcher to take it out as soon as possible. They take a few direct hits to destroy.

Soldiers will attack from three directions: both ends of the main road and a side road behind you. The wall provides cover, allowing you to move between its ends to deal with soldiers approaching from different directions.

If you find yourself getting overwhelmed by enemies, don't hesitate to use Trevor's special ability. This will give you a temporary advantage to clear out attackers on one side, creating an escape route. Prioritize escaping towards an area without a tank.

A really effective tactic for dealing with the tanks is to activate Trevor's special ability and then bombard the tank with grenades from your launcher. While his special is active, you can stay out of cover without the immediate threat of the tank's cannon. Make sure to save Trevor's special for these tank encounters, as they are your biggest threat.

Once you've destroyed the two required tanks, shift your focus to racking up kills and headshots. With the tanks out of the way, you should be able to clear the remaining objectives without too much difficulty.