Can somebody help with Halo 3?

What I would really like is to find the walk through on the internet for free. Can anybody help there? I am having a hell of a time where Master Chief is supposed to go into save the Marines, but is met with the Aliens. He can hide behind that green turret thing, but that big f@#ker with that hammer comes down and kills me. Dammit! How do you kill that SOB and help me get through this. My wife has become scared of me!!