After you survive the raid, you must choose a path to open; left or right. Open the right gate and it will take you to El Gigante. Use the shotgun and make the boulder drop down on his head when he arrives. Then, turn around and kick the door six times to knock the chains off it. If you are still standing outside the door when you see Gigante approach you from behind, do not panic; just get out of there quickly. He does not attack for a few seconds after he comes up behind you. The knife or a gunshot may help destroy the chains quicker, but kicking works. Go through another door in the same fashion and somewhere along the way you should find a key. It may be in one of the last huts you see toward the center. If you are quick enough, you can escape El Gigante and make it through the door alive with Ashley.
