Your next mission begins east of Costa Sud where you meet Mario and the President. You are tasked with protecting the rebel cargo against the plethora of enemies that attack. The easiest way to defend against this assault is to use your tether to move the heavy anti-air gun on the tower forward so you can use it to attack the enemies facing off against you.
You can basically take out all of the enemies in this mission using the heavy anti-aircraft gun to make it super easy to complete. Once all of the enemies are down, grapple onto the helicopter (once Rico catches up on his motorcycle) and plant some explosives while you’re hanging from the bottom of the chopper. When the helicopter goes down the mission is complete.