With this method you'll be able to stay AFK for 10 mins while you are getting the medal in each single map, so these are the steps to follow:

1. Enable the console from the Steam UI, using this parameter: -console
   Instructions: Select L4D2 in your Steam games list, Right click --> Properties and you'll find the "Launch Parameters" option right next to the "General tab".
   
   
   
2. Now open the game and as soon as you are in the lobby, you will see the console in there.
   
   
3. Type these commands:
   bind "c" "sv_cheats 1; noclip; sv_cheats 0"
   map c1m2_streets survival
   
   
4. Now pull the switch (Each map has a different switch) to begin the counter on the map.
   
   
5. Press "c" and hide in one unreachable building.
   
   
6. Press ALT + TAB and keep doing your stuff, then come back 10 mins later.
   
   
7. You can check your progress pressing "ESC" --> "Achievements" --> Scroll all the way to the bottom and click on the golden medals achievement.
   
   
   
8. To move to the next map use the Voting system: "ESC" --> "Vote" --> "Select chapter"
   
   
9. Once you are in the new map, you just need to do the same from the 4º step.

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If by any chance there's a map that is not counting your progress at all, just restart the game and try the map again.
Credits to: @Malik

- You can use this command to enable the console in the match:
bind "9" "toggleconsole"
(You cannot open the console in the lobby)

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To complete this you can use the mentioned noclip command to fly across any map until you are in front of her and kill her in one shot.
Use this command at the begining of the map:
bind l "sv_cheats 1; give adrenaline; give pumpshotgun; upgrade_add explosive_ammo; sv_cheats 0;"
Press "l" and start flying to reach the witch.

PD: You can use this video guide to check where the witch is.

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You can use these commands to complete those cheevos real quickly:
1. map c1m2_streets survival
2. bind g "sv_cheats 1; give knife; sv_cheats 0" - press "G" to get the knife.
3. bind H "sv_cheats 1; z_spawn smoker; sv_cheats 0" - press "H" to make the smoker appear and cut his tongue.

Now use these commands in the same map:
bind g "sv_cheats 1; give shotgun_chrome; sv_cheats 0" - press "G" to get the shotgun.
bind H "sv_cheats 1; z_spawn hunter; sv_cheats 0" - press "H" to make the hunter appear and kill it as soon as he jumps at you.