https://youtu.be/JbTztMJxuOc


1. Choose the map 'cbe_office'

2. Choose Technician class

3. Type this at console → sv_cheats 1; noclip; sv_cheats 0

4. Follow the guide video to direction

5. You have to place at the perfect place, unless you can't install the sentry gun.

6. Bammm now just play another game, do homework, help some chore, and you'll eventually get achievement.

I don't know when you place sentry gun under the table, game doesn't output the gun sound. But it doesn't matter. And Thankfully zombies cannot attack the sentry gun. So you just wait couple of hours behind the sentry, and achievement unlocked :D

Warning : Since the special zombies drop some ammunition, there will be some lag when you kill around 2000~3000 kills. Just restart the map and do the same way.