Left 4 Dead

This is a tough one two get. I think No Mercy is the easiest to do it on. Get together with 3 friends. Stick together and help each other out. Remember only one person has to make it to the safe house. My tip for part 5. There is a closet to the right of the ammo at the end. All four survivors get in. Two crouch down in door way and start melee as soon as you call for help. The other two stand behind and shoot the zombies being melee'd by your team-mates. When the tanks come run out of the building and throw a moltof at him and then run. Don't try and shoot him he'll catch up and hurt you. If he gets to close to a team-mate jump on the turret, he will instantly go for the person on the turret. You can also try to get him stuck on the ladder. You do this by climbing a ladder while he is following you and stop on the edge of ladder at the top. If he follows you will be blocking him from getting up and of the ladder. Just look down and watch him burn or soot him. Or your team-mates can shoot him. After second tank is dead get to the hela-pad the helacopter is here. If some one is down or getting pulled away LEAVE THEM. Only one person has to make and everyone in the room, even if idle, will get the achievement. WARNING!!! Be prepared for 3-7 hours of gameplay, because you will die alot and have to start at the safe-house again.

Expert is very easy if you can simply change the way you play. You cannot possibly enter an Expert game with the mentality of another difficulty.

Friendly fire - Don't even bother shooting if there's more than a 50% chance of hurting a fellow survivor in the process. It causes major annoying damage and can really hurt.

Zombies - The Common Infected do 20 damage on a front hit and 10 damage for a back hit. The best thing? They have to actually come right up to you to do said damage. Try headshotting with the shotgun, or using one of the rifles but always aim for the heads.

Play with full captions on. You will get many notifications as to incoming attacks and Special Infected spawns, but do note that you should listen for the music cues anyway. If you like to play with music off, at least turn it on a little bit. It really does help.

On Campaign mode, you can actually hear tanks BEFORE you trigger them. On Versus, they just spawn and boom, but no: on Campaign mode, you can hear a tank and it's only after walking past a door or something they are activated. This gives you time to prepare, and it's even possible to see part of a tank before it gets activated.

MOST IMPORTANT TANK RULE:
Setting a Tank on fire makes it FASTER on Campaign and activates a Fire Timer (something like 40 seconds on Expert mode?) before it dies. Any shooting you do when it's on fire is actually a waste of ammo (although sometimes, especially with a rifle or mounted gun, it slows it down a bit). Just try running to the saferoom if you've used your molotov or gas can. There's no need to shoot.

I believe this is the same for Witches, so do not bother shooting witches on fire either unless you like wasting ammo. I doubt it slows her down.

And Tanks and Witches can both enter the beginning saferoom, Tanks sometimes struggle and certain positions help prevent it but it is sometimes possible for both to enter (with a Tank, however, it's slightly more unlikely).

My final tip is to play with the controller sensitivity all the way up, both vertically and horizontally, in the pause or main menu. Takes a long long time to get used to, but you will find you can aim much faster and you have freedom of movement. And be aware! Always look behind you and in front of you - you almost need eyes in both directions at once.

If you're only aiming for this one, do a campaign you know the most thoroughly. Their difficulty is about the same.

There is actually a glitch to beat every finale without so much as killing a single Infected. On No Mercy you can let zombies break down a door and then go into a room where the AI can't see you, on Dead Air you can jump on top of the plane by moving one or two carts with propane tanks, on Blood Harvest you can actually just stand and melee a tree and everything will immediately die before it reaches you (!) and on Death Toll there's a rock you can sit on. Crash Course, as far as I know, has no finale glitches, however Crash Course on Expert doesn't count towards it anyway. But I'm not going into how they work and not saying you have to use them. It has little relevance, just find them out through Google or YouTube.

Don't forget you MUST SURVIVE the finale and ENTER THE RESCUE VEHICLE! It's for surviving a campaign, not only for completing one.

Hope this helps.

NO MERCY EXPERT DIFFICULTY TIPS

1.) you need all 4 players to do this job.
2.) No Mercy is the easiest campaign by far
3.)this might be obvious but stick together
4.)close every door behind you(except start of level safe room)
5.)tier 1 weapons(pump shotgunmini-uzi) have 3 people carry the uzi and 1 carry the shotgun
6.) tier 2 weapons(auto shotgun,assault rifle, hunting rifle) have 1 person carry the assault rifle, 1 carry the hunting rifle and the other 2 auto-shotguns
7.)have 3 people carry molotvs and 1 pipe bomb
8.)listen to some music(rammstein worked well for me)
9.)get some food, a drink and have a 5 min break in between levels
10.) be prepared for a longnight of gameplay but it is so rewarding as soon as you earn that achievement

OK it is a glitch and i respect that many dont like glitches but here it goes...

On no mercy there is a door that you cannot break down near the gatlinggun post have one person (or 3) stand above that doorway (climb up via ladder) and the other answer the radio. He/she should then move to the others guarding the ladder. A zombie will break down the door that you are standing above. go into this and stay there!!!

PROVIDING THERE HAS BEEN NO REPAIR PATCH!!!
You will not be harmed as long as there are no zombies in the room. They will not run in the room unless you go out. Tanks will completely ignore you and die off on their own...

just be ready for when the helicopter comes...boomers and hunters and smokers oh my!

Heres a simple way start a campaign on easy run through til the finale while in the safe room players that want the achievement / progression point leave to the main menu. The host must stay in then send invites to everyone to come back and leave the safe room when the players join they will automatically start idle (this will not happen if they are still in the same room) they have to stay idle until the end. When the rescue vehicle arrives switch to expert and have everyone take control of the players. then just run onto the vehicle and everyone who left and came back will get the achievement / progression point simple. If the horde over runs one of you just all die and you will start back in the safe house have everyone leave again switch to easy and invite back. Remember to return the favor for the host. It also pops off a bunch of them if you don't already have them akimbo assassin stand tall nothing special stomach upset there are a few more but i cant remember what they are.
This worked for me yesterday on 19/7/12 happy hunting

To clear up confusion and all that stuff: Only one person has to make it to a safe room to move on to the next chapter. Say in chapter 1, if everyone except one person dies and one person makes it to the safe room, then everyone will make it to chapter 2 and so on. BUT for someone to get this achievement and credit toward What Are You Trying to Prove?, he or she MUST make it aboard the rescue vehicle. That's why this and What Are You Trying to Prove? are so hard. If you are trying to help someone get this achievement and they die during the finale, then the best thing to do is to let everyone die so you can respawn in the safe room. If a person who needs this dies in the finale and the others make it into the rescue vehicle ONLY those who make it to the vehicle will get the achievement and credit towards WAYTTO (I had to make an abreviation bc it took too long to type!). So not only will they not get the credit, but they must do the WHOLE campaign on expert again... Talk about frustration. That happened to me a couple of times when I played with inconsiderate jerks in Blood Harvest (my last campaign I had to do on expert). Anyways, it all worked out in the end :)

I did the glitch for this one, if you are not familiar with this glitch, what you do is...
1: Set up a game on any campaign with atleast one friend on the easy difficulty. (I advise Death Toll as it has two different jesus spots).
2: Play through the first four chapters on easy, when you spawn in the starting saferoom, call a vote to return to lobby and all vote yes.
3: When in the lobby, change the difficulty to expert and start the final chapter. You have to survive this chapter on expert, this is why I advise doing it on Death Toll, it's the easiest and has two jesus rocks either side. Remember to incap your bots before contacting the rescue vehicle as they will mess up the jesus spot. I'd advise the rocks furthest aways from the pier as they are more effective and the tank dies instantly were as it still finds you at times on the rocks on the other side. Make sure you both have pipe bombs and as soon as the boat comes, run, use your pipe bombs and this achievement is yours. If the glitch has been patched then I'm afraid you will have to do it legit, try and gather some very experienced players.
PS: I strongly advise that you do NOT try and get the "Untouchables" Achievement while doing this.

Okay, I feel like this solution needs some updating.

All you have to do is play through No Mercy on Easy until you reach Part 5.

Once here the host needs to initiate a vote to return to lobby. Once in the lobby, change the difficulty to Expert and change the level back to Part 5.

Then you need to gather at this point -> HERE
Ignore the part where you Molotov the side ladder, it's not necessary.

It's recommended that you don't kill the zombies that are already spawned when you initially climb to the top of the buildings from the elevator shaft; you can use them to your advantage before activating the radio. Now go to the ledge (so you are able to see the door) and wait for a zombie to bang on the door and break it down. Once this happens everyone jumps down and enters the room (one person activates the radio, then returns to the "Jesus" room). Once in the room, put your backs to the wall and wait; the Tanks will kill themselves after awhile. Once this happens (ten minutes or so, there is two tanks) you will exit this room and head east. From here you need to make it to the helipad and leap on to the helicopter.

All of the survivors get the achievement.

Credit to Crimson Carmine for the video

I actually got some of these coincidentally when doing the leave and rejoin method to survive an Expert Campaign. Because of that we attempted to reproduce it and found that this method works for these Achievements. It would appear that you can at least earn these Achievements while using the same method that people do for Zombicidal Maniac, and What Are You Trying to Prove? (As is shown here Solution for Zombicidal Maniac In Left 4 Dead):

You can at least earn these, as we did these this way

1. Zombicidal Maniac : Survive any campaign on Expert.
2. What Are You Trying to Prove? : Survive all campaigns on Expert.
3. Safety First : Play an entire campaign with no Survivors taking friendly fire damage.
4. Untouchables : No Survivors take damage after contacting the rescue vehicle.
5. Nothing Special : Survive a campaign with no Survivors taking damage from Special Infected.

The Method

1. Play through the Campaign on Easy until the final Chapter
2. Whoever needs the Achievements leaves before opening the Safehouse door on the final chapter.
3. They immediately rejoin but just Spectate as AI.
4. Everyone else gets to the very end, right before getting in the escape vehicle, like RIGHT outside it.
5. If going for Expert based Achievements, call a vote to switch the difficulty to Expert.
6. After the difficulty changes (for Expert based Achievements); the people idling as the AI that need the Achievements rejoin and everyone very quickly gets in the vehicle without any damage from any source.
7. The people who were idle should get those Achievements. We were able to reproduce it 3 times for different Achievements; a lot unlocked at the time time, too.

Here is a video example from our Session, recorded on an Xbox 360 on 2022-03-15; I'm sorry about the doubled Audio, I didn't know until we were done. Unfortunately I actually don't get any Achievements to pop here, because I already had them. At the end you can hear one of our participants say it did work for him, though.