Whats the best way to shoot down the helicopter. without other co op help i can only get that far then get belted. any ideas?

M107 or a SAW. Of course, then you are limited, as neither are as versatile as a rifleman if you are playing alone.

You can shoot it down with a rifleman weapon, but it will take a bit longer.

if you hurry and turn the corner fast, you can take out the coptor while it's still on the ground.

I took it out with an MRC. Use one of the rocks as cover and just walk round it as the chopper flies over. Shoot at it when it banks and has its belly showing.

use an m60 heavy machine gun and when its down use an enemys carbine thats is on the ground.