Below is my video walkthrough of this act (on realistic). The level itself shouldn't be too challenging if you've managed to reach this stage on realistic, but the final sequence with the chopper can be unforgiving.
Just try to listen closely to the chopper's machine guns. As soon as they stop, you know that it is about to fire a missile. In which case you need to run to the side of the fence that the chopper is not facing.
Grenades can also be a pain. If you can use a sniper rifle though, you may be able to kill the enemies off before they come in range to be able to throw them at you.