When you get to the area where there’s a few enemies and one of them is holding a Javelin you’ll want to take them out. In this room there are two Javelins, make sure to pick up one in the right corner. Then you will want to pay attention to the green arrows that are on either side of your screen as it tells you which direction the helicopters are flying.
Before getting on the Sniper make sure to have a good look around in the sky because chances are is that there's a helicopter that's flying behind the building you are in. It will be on the edge of the map flying really far away. Try locking onto that helicopter and eventually the rocket will hit it.
You then need to get on the Sniper Rifle to snipe the enemies on the other side of the area. Then some enemies will try to breach the room you are in so take them out. After this, some more helicopters will spawn so take them all out with the Javelin. Once enough helicopters have been taken out the game will make your Javelin disappear.
You will want to pick up the other Javelin that the enemy was holding earlier in this room. Whilst the escape timer is going down keep on locking onto the remaining helicopters. Eventually you’ll see no more helicopters.
If you are still struggling, feel free to use this as a reference.
Hope this helped, thanks again.