All co-op should be played with 4 players for the easiest win.
Difficulty -- This is the 2nd hardest of the 3 co-op missions.
At the beginning of the valley, there are at least 3 enemies directly ahead of you. You have the option of taking them on up front, or go to the right of where you spawn and climb the vines to grab a bow.
As you proceed up, you will come to a large tree trunk. At this point, do not cross the entire length of the trunk until you defeat some more enemies. Once that's clear, I suggest switching to your knife and running straight across the length of the trunk, all the way until you hit a rock wall, without stopping, as there is a rocket launcher armed enemy and another enemy directly above that rock wall. There will also be a weapon at the end, so if you just camp there, the rocket launcher guy will not be able to fire at you. Sniping him with the pulse rifle or bow while another player tries to distract him is ideal.
Just remember to take your time as you progress.
Once those 2 are dealt with, cross back over the length, but watch your left as you'll notice the path goes higher, which is where you want to go. You'll face more enemies here, including one mini-gun armed enemy. Take his minigun as you can always use it as a sentry turret.
Clear the way and proceed forward, but watch the shrubbery as a dino will come through. After him, there will be some enemies to take care of AND A SNIPER in a tower at about 10 o'clock from where you enter this area. Do not proceed until the sniper is down, and you should hear some alarms during this.
If you take the low (left) path, as you make the bend to the right, PROCEED CAREFULLY as there is at least 1 turret manned that needs to be taken out. The high (right) path is more vulnerable to the turret but might have better access to killing the turret enemy.
You'll now be at the gates and ready to hack the two terminals. Take a minute to set up your minigun turret so that it can view at least the right side of the next area.
This next area has a ton of enemies, and all I can suggest is to have 2 players plus the turret watch the ground troops, while 2 other players watch the upper right (about 2 o clock) walkways for snipers and enemies. As it starts to die down, a player can start trying to hack the terminal but DON'T HESITATE TO STOP AND RETREAT FROM HACKING if you start taking fire.
Once that's complete, you will have new waves of enemies spawn from where you first entered this area (the gate), and from 9 o clock (they will blow a hole through the wall). There is at least 1 rocket launcher enemy (at the gate you entered this area from).
Clear them out, and proceed to 2 o clock where there is an opening back into the valley, with dinos and at least 1 dilo.
Clear the way, proceed ahead, and proceed carefully.
The next few areas will have at least 1 sniper at every hack terminal you need to hack (to align bridges), and you will also pass 1 or 2 minigun enemies that you should again have players pick those weapons up (for the end).
Make sure you never proceed until all snipers are accounted for.
There is an open area also with a large tree trunk, with enemies spawning below (and 1 on the trunk), so make sure you thoroughly clear that area out.
The last area has some cool weapons to pick up, and have 1 player pick up the rocket launcher against the left wall.
DO NOT RADIO HQ UNTIL ALL PLAYERS ARE READY AND SET UP.
The end consists of staying alive for a set amount of time while dealing with waves of enemies and dinosaurs. Set up your mini guns and try to take cover (at least 1 player, spreading out helps spread the enemies attention) along a blind spot which is 2 o clock from where you enter this area. It's just along the right wall if you follow it to the edge, and enemies will not be able to see you directly without moving in closer.
I set up 1 turret right there and camped that spot with a rocket launcher, while my 3 buddies clung to the opposite wall (to the left or 9 o clock from where you enter this area, where the rocket launcher is).
Now, enemies will spawn from where you first enter this area (Wolf Pack, at least 1 rocket launcher enemy per wave, and Dinos, from 3 o clock from where you first enter this area [if looking out over the cliff]).
If positioned like I mentioned, have the buddies along the wall by the rocket launcher watch straight ahead of their position for approaching dinos (AVOID THE TEMPTATION TO KNIFE AS THE ANIMATION TAKES TOO LONG) and remember that they must not stop clinging the wall unless they want to be rocketed (until the rocket launcher enemy goes down).
Eventually, 2 DILOS simultaneously appear (about every 3 waves or so) and this is where your rocket launcher comes in handy (especially if you're hiding at the blind spot).
Just survive the time and you win.