Ok, I'm at the part where I just opened the gates for a tank full of the good guys, all of the sudden this helicopter comes by and starts shooting me. I shot it with the rocket laucher and hit it with all the rockets I had. It still didn't go down and everything I try doing doesn't work. It wont let me go to the objective until I get rid of it. What do I do? The part is called Gate Crashers.