Once you shoot down the helicopter and the door opens and 4 more swat men come in...where is that arrow prompting you to?? There is a message telling you to pick up any useful materials but when I follow the arrow back to the room where I shot down the helicopter...there is nothing.
Looking at the helicopter it makes a pass to the left while firing but when it passes to the right it doesn't. Hide on the left or right side walls that are tan colored or the statue in the middle. On the non-firing pass shoot in first person. Third person would need you to get close and you need to constantly face the helicopter. After the non firing pass 4 SWAT will rappel in. Take their weapons. Just disarm and beat them up, you have no health to spare going into a gunfight with them. After this the cycle repeats itself with the helicopter firing again until you kill it. After the helicopter goes down several swat will attach from the adjacent door. Hide behind the statue and engage them before continuing the level.