ALL CREDIT TO "Wabbytax" From xbox360achievements
This Help A LOT for a friend of mine.
Make your way through the caves like normal until you get to the ledge that starts the cutscene. As they say they will cover you turn around and kill the infected that jumps down behind you, then walk back into the corner about 1 meter behind you and start picking the infected off as they come towards you. The marines up top do ALOT of the work for you and after a while all the infected are dead. Continue walking to the back of the cavern and then sprint to the red smoke since infected will still spawn and you should make it to the marines in time for the end of the level cutscene.