Fallow comrades, go past the corridor and up the stairs.
You'll reach the gigantic cave, with stairs in the center. Run up (try to do it fast), where defence point is located (place between a few destroyed walls).
Pick up some ammunition from the hanging creates (lying on the ground - sniper rifle, hanging - shotgun). Hayes will grab ammo too. Get ready for the fight with many dinosaurs. The attack will start in a few seconds.
The enemies will show up from. below, but from different directions. Stand on small raising with some bones nearby (picture on the right) and start shooting. In this way, you won't have to turn around all the time. Jump down only when your comrades will ask you to, but quickly return back. You can also approach attackers (shots will more effective). Remember about having loaded bullet all the time (take advantage of short pauses between the attacks). To shoot down the dinosaur you'll need three hits, so try to save some ammunition.
After a while, new (never seen before) reptile will start attacking you (fading music and sounds of steps will inform you of it). He'll be something between your current opponents and tyrannosaurus rexes. New enemy will fight with other creatures in the beginning, so take advantage of the situation and get rid of him quickly.
When the fight will end, go through the gate, which you have noticed certainly already. Unfortunately, the road is blocked by the shrubbery and there isn't any handle to open it too.
Ann will inform you that she noticed tunnels, where you can find needed handle and fire.
First, enter the one on the left (screenshot). At the road's end you should find useful fire.
After going past the first tunnel you'll come across the small room, where you'll find entrance to the narrow path, taking down (destroy the blockade first). The lower part will be flooded.
During swimming, you'll be attacked by the crawling worm so eliminate him (shoot). You'll reach the bigger room, pass it and...
...climb on the 'island' with the bait. Take one of the worms (spear) and throw it ahead, to the next cave (with water on the surface). Some enemies will fall in your trap, so get rid of them with the rifle.
Go through the room and enter the next tunnel.
You should reach the goblet covered with shrubbery (left screenshot). It won't be surrounded with wall, so you will be able to throw fire from nearby chamber. Move ahead, through the waterfall.
Stand on the top to see an small island lighted with fire. Before you'll go there, eliminate some crabs trying to kill you.
Run down now. From the island's centre you should get fire. Fire the spear up and throw it into the goblet passed earlier (right picture). When fire will spread on the shrubbery, move up again, through the waterfall. Try to do everything very fast, because crabs will attack you after a while, and there is no sense to kill them.
Along with Ann, set all goblets on fire. In the room, where you have used the bait, some crabs will be waiting for you. Eliminate them with the rifle. You can have problems with the worm in the flooded room, but use the lighted spear if you'll be able to return for another one.
Now, after destroying the shrubbery blocking the way, it's time to find the handle and open the gate.
The way is leading through the tunnel, behind the waterfall. Move there.
When you'll enter the tunnel, you'll see the dinosaur and two worms running in your direction. They'll start fighting with each other, so take advantage of the situation to eliminate them with ease (first, they'll attack and then eat one another).
Walk ahead to the place lighted up with fire, where a hole in the wall is located. Approach it with the lighted spear.
You should see the glade, where two dinosaurs are. Do not waste ammunition, just throw the spear in the grass to fire it up and 'fry' the beast.
Jump down and quickly pick up the desired handle from the pole knocked over at top level of stairs (some worms are moving in the neighbourhood, but if you'll be fast they won't manage to attack you).
Now, return to your comrades, taking the same way. Put the handle in correct place and open the gate (finally).
Go through it and finish the stage by entering the 'oasis'.