On the level right after the Cairo Station, Sargeant Johnson is invincible. He is the black dude with the sniper rifle, the one that says "Fear humanity, we regret being alien batards, we regret..." and so on. You can use this to your advantage, let him do all of the work. Unfortunately, he eventually boards the pelican almost right after you kill the pair of hunters. Oh, use the turret on the building to kill the hunters very quickly, unless you are playing legendary in which case they will probably blow it up.