You have to protect the oil barrels and the dock boss; thankfully you're given a SAW for this mission. Take out those coming via the road and once they're under control, turn around and take out those coming in from behind. After those, reinforcements will arrive by helicopter on the platforms. Don't enter that area or they'll completely surround you. Instead, wait for them to come to the starting area and take out all that you can see. You should be able to take out quite a few while they're running along the higher level. The final wave will come via boats, so head over to the side and shoot those coming up from the dock. Now turn around and take out those few stragglers coming in from the other directions.