Hey! Welcome to Easy Steps : Remove all special infected during campaigns.
I will show you how to remove special infected easily. Without cheats, so you can still gain achievements.

First off, Turn on console,
You can turn console on by going to > options > keyboard > developer console > enabled.
or, alternatively > go to steam > library > left 4 dead 2 > properties > set launch settings > type "-console"
this will allow you to turn on console.

Second, Choose campaign,
Simple, play any campaign.

Third, Open console and type this commands,
Now the part you all been waiting for, so open console by pressing " ` " under the escape button,
then type in

alias loop "kick Tank;wait 1;kick Boomer;wait 1;kick Smoker;wait 1;kick Hunter;wait 1;kick Jockey;kick Charger; kick Spitter; kill Witch;loop";
loop;

to stop, just do the same but remove the "kick tank, bla bla etc" or do this,
alias loop "";
loop;

Fourth, enjoy!
Thank you please rate and favorite this guide if it's helpful! See you in the apocalypse!

Update:
It seems like Witch cannot be removed, so stay vigilant!
Another Update
If you want to remove the Witch, make sure to type "sv_cheats 1" then the command. Thanks, and have fun!