You'll start the level in the place, which I called oasis before. Two crates with ammunition are located in front of it. You should see the first one, the second will be separated with the shrubbery.
To get to the crate, pick up the bone lying nearby and fire it up at the fire in the building. Throw bone into bushes. Take the ammunition to sniper rifle, which will be useful after.
In this stage, fights are going mainly in water and you'll attack all time by the monster living there. You haven't seen these creatures earlier (something like snakes on four paws). When they'll attack, you should be able to see and hear the splash of water.
After few meters passed, you'll have to enter the water for the first time. As I mentioned, get ready for a fight. You'll attacked only by one enemy.
When you'll pass the first section of water, move through small island stand in front of the water container again. It'll be bigger and more dangerous (more opponents there and one in the air). Do not enter water, simply let go your comrades first. Cover them from the attacks. Use the sight to zoom and shoot when somebody will be grasped by leg. You can also kill the monster, showing you their position by splashing the water.
When you'll be standing on the other side, enter the pond alone. This time Hayes will be covering you and you don't have to worry about your life, because he's a good shooter. Try only to move away from the monster for a while, so Hayes will have more time for precise aiming.
Next land, small tunnel and again, you have to walk in the water. This time it'll be longer journey. During passing the first section of the swamp (the oblong one), four monsters will attack you, and when...
...you'll reach the bigger reservoir one hell of a fight will start (many lizards attacking from different sides). Don't save ammunition but shoot as fast as you can. Some flying creature will show up, but shooting him down will be useful - when his body fall, it'll eliminate a few enemies. After the battle, head to the land (right picture).
Pick up some ammo from the crate. After few steps, kill three 'bats' hanging under the branch (you'll pass it after a while, so you wont' be forced to fight then).
Taking the next, short passage between rocks, you should stand on the bigger glade. Lightly destroyed wall is located in the centre of it. Climb up and head to the bigger platform.
Take advantage of lighted spear and set the shrubbery on fire to clear the way for comrades.
Now, turn left and start killing enemies moving in water. You'll cover colleagues this way, because they won't make it on their own. The way is long, so try ain precisely and don't waste ammo for nothing. Move ahead when your friends safely reach the other side. Hayes will be covering you again.
When you all find yourselves on dry ground, go up the stone stairs. Enter the cave and walk upstairs again. The level will end.