Passing the small section of the way between rocks, you'll again reach the canyon with moving brontosauruses. Your comrades stand on the other side of the precipice. Carl will try to open the gate and Hayes will kill some monsters. Use sniper rifle to help him, or they won't wake it on their own.
When your friends go ahead, walk in the direction showed on the left screenshot.
At the road's end, you'll reach the wooden bridge. When you get near it, some flying creatures will attack you. Don't fight, but simply run to small room with windows, located near the catwalk. Take advantage of the sniper rifle (the view from the windows - right picture).
Move on the wooden construction, until you reach the crate with some ammo. You'll hear your comrades shouting from below. Of course, they'll need your help, so kill crabs that show up from under the closed gate.
Move ahead to the tunnel. You should reach the bigger chamber. Stand at it's entrance (make sure that no monster can see you) and start the extermination. Take advantage of worms fighting with themselves, hanging 'bats' as bait and fresh meat lying near the stairs.
After the fight, walk down.
Approach small wall, which divides the room in two parts. Do not destroy the wooden gateway, but kill next opponents through the gap located close by.
Now, move there (through the waterfall) and walk ahead.
You'll be passing the next section of the tunnel, where small windows are placed (Carl and Hayes will stand behind them).
You'll reach to next bigger chamber. Move down stairs and run into crabs leaving water. In the half way down, you should find some ammunition. On the other side of the room, the gate is located. Your comrades will approach it after a few seconds, so move there and (together) fight off new attacks.
After the fight, you'll have to find missing handle to the gate's 'lock'. It's lying behind shrubbery at the beginning of the chamber, and you have to had fire to destroy the obstacle.
To get it, return (long way) to the place situated in front of the waterfall (two worms was fighting there). Look at left picture and you should see fire container located under the ceiling. Shoot it down and you'll be able to fire the spear up.
Now, you have to avoid the waterfall. Remember the place where you've to kill some worms by taking advantage of the hole in the wall... throw spear there to the next chamber (screenshots presents the whole process). Thanks to it, fire is located on the other side of the waterfall.
You'll only need to fire 'the torch' again and then get rid of the unnecessary shrubbery. Pick up the handle and place it into the pole opening the gate.
Go through the gate to finish the stage.