There are some guides out there already that you can follow without downloading anything which makes this guide focus on other hard to obtain achievements.

Here are the guides that will help you on the other achievements that i wont be explaining how to get here:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3010503028
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=894028894

By taking advantage of the developer console getting achievements are much more easier to obtain in solo so make sure to turn on developer console in:
OPTIONS > Keyboard/Mouse > ALLOW DEVELOPER CONSOLE > Enable

And also rate me if you find this guide helpful so the others can know!

Legendary Survivor is one of the hardest achievement to obtain in L4D1 because the game is dead and we sometimes have no friends to help us, so to set things up you will need to do these steps:

• Go to LIBRARY and find Left 4 Dead right click to open manage
  
• Click Properties
  
• In General scroll down and you will see LAUNCH OPTIONS
  
• Put in LAUNCH OPTION +sv_pausable 1
  (this will let you pause the game in Survival)
  
• Launch the game

After you did the first step go to Survival and create a lobby choose whatever campaign and switch the SERVER TYPE to Local Server

When your in the game press "~" to open developer console
put in sv_cheats 1; noclip; sv_cheats 0
(it wont disable achievements cause it will turn off after noclip is enabled, and noclip will still be there)

After that noclip outside the map or hide inside buildings so you cant get attacked by special infected's and wait until 10 minutes

Reminders: your time wont be registered but it will be counted on the MY ACHIEVEMENTS that you got a gold medal.
Now just repeat STEP 3 until you get Legendary Survivor! once you are finished just remove +sv_pausable 1 on said LAUNCH OPTIONS

There are other ways you can use "sv_cheats 1; noclip; sv_cheats 0" you can try it out for yourself for example skipping all the way to the next chapter in campaign and typing it again to turn it off before entering the next area, or whatever methods you try to get the achievements you wanted

You can literally solo an entire expert campaign for the "Something to Prove" achievement
by typing this command in developer console:
bind "MOUSE1" "+attack;kick Boomer; kick (1)Boomer; kick Hunter; kick (1)Hunter; kick Smoker; kick (1)Smoker; kick Tank; kick (1)Tank; kick (2)Tank; kick (3)Tank"

What it does is remove the special infected only in campaign mode, eliminating the need to download infinite kick script add-ons from sketchy websites. This method is much safer and easier.

You don't have to enable sv_cheats just put it in and BAM no more special infected!
what it does is every time you click the shoot button it will remove the special infected so you have to constantly click and not hold to remove the special infected, this takes practice and you will get the hang of it.
Or you can replace the bind key with any other keys for your own preference by doing this for example:
bind G "kick Boomer; kick (1)Boomer; kick Hunter; kick (1)Hunter; kick Smoker; kick (1)Smoker; kick Tank; kick (1)Tank; kick (2)Tank; kick (3)Tank"



(To restore it back to normal go to Menu (ESC) > OPTIONS > Keyboard/Mouse >
EDIT KEYS/BUTTON > Use Defaults (This will reset the keys back to default so if you have custom keybinds keep note of it before you put it back to default) then > Ok > Apply)





This is all for this Guide, good luck achievement hunters!