Secret Service

While in the helicopter you will get three checkpoints before finishing, use these to your advantage. Focus on the guys shooting RPG's first, the others wont damage you as bad! At the first check point try to have over 75% if you drop below that then reload last check point and try again. The 2nd check point should be around 60% if not reload check point. After the 2nd check point if at any time you drop below 50% reload checkpoint. Remember to focus on the RPG's first and everyone else second. Just keep holding the RT, unlimited ammo.

Adding to the above solution it is usefull to know that whenever you reload a checkpoint no matter the damage you received at the start of the checkpoint the game gives you back around 75% of the armor. So do not mind how much armor you have at any checkpoint just make sure that whenever you are below 50% reload again.