Work your way through the tunnels, until you get outside to the forest. Continue on the trail, watching the ground until you get to the bunkers. Traps will trigger QTE's.
Take up position behind the leader, towards the left side facing the bunkers. Just camp everyone. Ammo is down in the pit. Once the cutscene is triggered, run to the makeshift stairs to trigger the cutscene to get into the bunkers.
Make your way through the trenches, being mindful of VC in the upper echelons. Eventually, when you get to the bunkers, you're going to explode and get into ANOTHER flippin cutscene. Take up refuge opposite diagonally the big radio tower and take people out as they came in the beginning. After the first few minutes you can move around the room.
DONT LEAVE THE ROOM UNTIL THE NEW OBJECTIVE APPEARS AND A TRAIL OPENS UP!
Almost immediately after, you'll be on a mounted gun. Keep the VC away. The goal is to prevent them from getting into the trenches. The ones that spawn close on the right can be tricky, so mind them.
Afterwards, continue till you have a QTE with an infected. In the next scene, take up hiding in the doorway on the right as you enter the scene. Grab ammo and get ready for a sharpshooting session. Once they stop coming opposite your aim and to your flanks, pop out, shoot one, repeat. Eventually the doorway you hid in will open up into a tunnel.
PART DEUX
The tunnels are pretty linear, so just continue until you get outside. Headshot any zombies in your way and make a mad dash to the end of the jetty on your left.
Achievement Unlocked!