Difficulty - this is the easiest of the 3 co-ops and this should be the one you spam when boosting for All Purpose Warrior.
Grab the bomb and proceed along the rock (take the high route) but as you approach the fallen tree trunk, watch for sniper at 12 o clock. There's only one. There's a bow here so you should be able to dispatch him quickly.
Once he's dealt with, there will be enemies and dinos along the ground and also at the gate and above the gate. Either wait till you kill them all or sneak by a successful bomb plant to end this area, and make sure you RUN FAR BACK away from the bomb, as it has quite a large blast radius.
There's about 5 enemies at the apc in the next area, so clear them out, but as soon as you notice the objective change, take cover as there is a sniper in a tower at about 12 o clock from where you enter this area. Pick him off and before you pass the APC, hug the left wall as there is a rocket launcher turret and even a rocket launcher enemy at 10 o clock.
Clear the ground troops and have 1 player snipe the enemies at the turret from above the rock wall (you can access if you circle around the APC and take the stairs). NOTE: THERE ARE MULTIPLE SETS OF ENEMIES THAT SPAWN AT THE ROCKET LAUNCHER TURRET SO DO NOT PROCEED UNTIL THE OBJECTIVE CHANGES, TELLING YOU TO HACK THE GATE TERMINAL.
Once that's clear, the objective changes. Take this time to grab the sniper rifle from the sniper tower, and also the rocket launcher if you proceed across the top of the next gate (when you get to the rock wall after you move across the top of the gate, there are vines you can climb, rocket launcher on the left once up there). When you're ready, hack the terminal and have someone man the turret to quickly blast the next enemies that will spawn behind the gate that opens from hacking the terminal.
The next area is where you get the boomstick achievement, and if you hug the left wall, you will not be attacked by any dinos.
There are a few small dinos, and 1 dilo appears after enough kills. After he's dead, another dilo will enter. So you have two chances for boomstick. Clear the area, and wait for the helicopter to blow open your path ahead so you may proceed.
This final area has at least 3 snipers, positioned at 12 o clock, 10 o clock, and 11 o clock. One enemy is also on the rocket turret at 10 o clock. Make sure you tag all 3 snipers and the turret enemy before proceeding.
When you get to the area with the helicopter, clear the area and your objective will change. Position a sniper above (you can get above by searching the area and going up a ramp that will lead you to an elevated area [there is more than one] where you initially killed the snipers at the beginning of this area), and have everyone else cover the helicopter.
If you're facing the helicopter, the next objective will spawn waves of enemies over 4 minutes from 12 o clock, 6 o clock, 10 o clock, and 8 o clock.
Make sure you're listening for the sounds of exlposions and looking for brown smoke as these are the triggers to show you where the enemies will be coming from. Waves 10 o clock and 8 o clock sometimes spawn together.
Things to note: 12 o clock wave (behind the tail of the helicopter, above the rock wall) usually spawns 1 rocket launcher enemy, so have the sniper watch for this guy.
10 o clock and 8 o clock waves usually spawn 1 sniper, so have your sniper watch for this.
Just survive the time and you win.