This mission was a bit of a bitch. When you get to the 2nd greenhouse, the helicopter will waste you dead about any time you're in view for more than a second. There's a few guys to kill, then when you get to the barn, kill one guy coming at you, then run behind that tractor. The helicopter changes its path based on where you are, so if you're waiting outside the barn a bit too long, you're dead. Once behind the tractor, your mates should come in and kill the one or two guys in there. That gets you to a checkpoint, then you can do what I did here. I am pretty sure you have to hit the chopper twice.
![](https://i.ytimg.com/vi/YnGKTNCI9NQ/maxresdefault.jpg)