This guide is only for the boss battle, as the rest of the mission is straight forward.
You cannot hurt the boss with your weapons; but there are a couple of high water tanks standing in the area. Make sure he sees you and wait til you notice he will be under the tank any moment; then call the helicopter for pick-up, preferably to a landing spot not directly where the enemy is. There are three landing spots on this battlefield.
Do not call too early though, because when the enemy sees the helicopter he will shoot it down.
After you called, fire a Missile on the water tank and the enemy will get a good shower and be unconscious for a little while; run to the landing zone and jump on board the second the helicopter is there. The boss will stand up soon and fire at the helicopter, and he can easily destroy it with two hits, even if you are sitting inside and taking off already!
The time is of the essence, that is why I recommend calling 5 - 10 seconds before blowing up the watertank; this way the enemy will not see the helicopter, but the lenghty fly -in is shortened. You could probably make it by calling after blowing up the water tank, if you have a pretty high support level. But I was sht down twice by the boss while sitting in the copter, despite calling the moment the water rained down on him.