Kill 53,593 scientists.

Enable dev console in settings
Host a game with the following settings:

Mode: Rat Race
Level: jb_coast03 ***
Map Time: 5 mins
Max Rounds: 1
Bot 1
Difficulty: Normal < important!

*** I found this map to be the fastest, but you can try others.

Once in game, open dev console with tilde key (usually the one next to 1 on your keyboard)

Type in the following into the console
bind c "jb_admin_spawn monster_scientist"
Press Enter.

Now everytime you press C, a scientist will spawn.
Set an auto hotkey script to spam C.

Once the round begins, walk forward out of the tunnel and look for the left-side railings on the street.


Here's the tricky part: when you click with the chainsaw it pulls you toward whatever surface you are hitting so stand in front of the railing, looking at the grass:


Now look straight forward and turn on your script. Scientists will spawn a little too far, slowly bring your aim downwards and eventually you'll find a spot where scientists will be instantly gibbed and respawned.

You can't fully automate it since your aim will jitter while you use the chainsaw, constant corrections are needed to find the "sweet spot", but this was as far as I know, the best way to do it.

SUPER IMPORTANT
You have to wait until the end of the round votes windows comes up or progress will not be saved. Once you see the voting screen you can safely rehost (faster than waiting).

Special thanks to Sunflower for their invaluable contributions to this guide. You da MVP!

If you have questions or find a better spot don't hesitate to comment below.