General things to know

• Blood Harvest uses random weapons distribution to full effect again! Only this time, weapons are often hidden in little side areas or not even present at all. There's a reason why this is the hardest campaign!
  
• Prepare for another joyride through the woods! This campaign has the most forested areas out of all the campaigns, and it's a real pain. Use pipe bombs judiciously to get rid of Infected hiding in the trees, and use Molotovs to kill off Hordes.
  
• There's a relatively low amount of confined area in this campaign, which makes it easy for Infected to flank you. Make sure to always cover your back as you advance forward.
  
• This campaign, more than any other, advocates having groups of two survivors taking the high and low paths each to move through the campaign. They can spot things the other group can't in most situations, and the paths are usually pretty close to each other, making it easy for one group to come to the aid of another.
• There's no car alarms, or metal detectors in effect here, so don't worry about what's in the background of where you are shooting.
  

Stage 1: The Forest

Walkthrough: Welcome to the hardest campaign of the four available with Left 4 Dead. Make your way through the forest until you reach the rope bridge. Make your way across, search the house for supplies, and make your way along the ridge next to the train tracks. Once you navigate through this area a bit, you'll run into a safehouse inside the train station.

Horde Choke Points

• On the rope bridge. They can only come from two directions and the area is narrow, making it easy for survivors to cover each other.
• Inside the house by the rope bridge. Two entrances and plenty of ammo makes for the best choke points.
  

Tips and Tricks

• The house after the rope bridge usually has ammo and another pistol to stock up on, and sometimes pills as well. Extra pistols and Molotovs also spawn near a break in the railway fence along the cliff as well.
  
• Pipe bombs are your friend in this campaign, as is sticking to one side or another to avoid taking hits from both sides in case of a Horde attack.
  
• It's easy to confuse survivors for Infected in the wooded area, so watch your fire!
  
• When you see the safehouse, run to it as quickly as possible. At normal pace, a Horde spawns immediately before the safehouse, and it can make you restart the level if you are caught unaware.
  

Stage 2: The Tunnel

Walkthrough: You start off inside the train station service area. Make your way down to the tracks, where you will set off an emergency alarm and trigger a Horde. After taking care of them, follow the tracks through the rubble to the next safehouse. Relatively short, but with a lot of Infected in the way.

Horde Choke Points

• At the top of the fallen yellow beam. This is the best place to fight the Horde that spawns from the emergency alarm, as setting the ground in front of it on fire is a great way to eliminate most of them. Watching to the left and down the ramp is all you have to do!
  
• On top of the train cars in the tunnel. Infected have to climb up to reach you, making them easy pickings for your team.
  

Tips and Tricks

• Don't stand on the ground floor during the alarm Horde. You will die, guaranteed.
  
• The Tank normally spawns inside the generator area or in the track tunnels. He can be a huge pain in either spot, so try and lure him into the open where you can dance him around without taking damage.
  
• The Witch spawns in the train station or on the generator room catwalks in most cases. If you see her and the Tank together, you can get them to attack each other by having the Witch run into the Tank as she chases you. Awesome!
  
• Boomers love the tunnels, and for good reason. There's tons of cover, letting him sneak up relatively easy on survivors. Make sure you watch carefully for him.
  
• Dual pistols and a grenade can usually be found in the room to the left of the spawning safehouse. It seems trivial, but they do help when fighting the Horde later on.
  

Stage 3: The Bridge

Walkthrough: Oh boy. Another forested section! Joy of all joys. Make your way out of the safehouse and through the forested hills, heading to the next maintenance corridor. After going through, follow the tracks until you reach a couple of derelict train cars. Decouple one to access the next area via broken bridge, but be warned that it alerts a Horde. After making your way up the bridge, follow the hill to the safehouse inside a shack.

Horde Choke Points

• Inside either of the maintenance corridors. The Horde comes in from outside, making it easy to pick them off as they run in, and the maze-like confines make leading them around pretty simple.
  
• In the building by the train decoupling section. In fact, this is where you should hold out for that section, as there's normally ammo and pills inside, and it's easily defensible.
  
• On the collapsed bridge, for the same reasoning that the rope bridge in the first level is great. Two directions and confined spaces make it easy to kill Hordes.
  

Tips and Tricks

• This level is pretty similar to the one immediately after it, unfortunately. All those wooded areas really suck for fighting Infected.
  
• If you feel daring, you can follow the train car as it crashes and climb up the bridge as quickly as possible, luring the rest of the Horde into a killzone. It requires incredible teamwork to pull off, though.
• Tanks normally spawn near the decoupling car, which can ruin your whole level strategy. Interestingly, Witches don't spawn all that often on this level.