TL; DR: Make a separated save, change difficulty to easy and go nuts at Colorado Springs, kill everybody exception of the travel points.
Preparation: first make a separated save, you don't want to screw up your game (or you do?), change difficulty to easy. Second, change your team! Some recruited companions will turn against you after the first killing (Lucy and Kwon confirmed). So, I recommend using 4 player custom characters plus Fishlips and Ironclad. If you don’t have them, try others (didn’t test) or you can go with the 4 player’s, it’s manageable.
UPDATE: Scotchmo will also attack you when killing refugees. Victory will be ok.
Now go to Colorado Springs via Ranger HQ (I recommend that entrance for organization) and start the killing spree!! The citizens won’t fight back and will one shot die, marshals will shoot you back, but on easy not too harm and they die one or two shots. Be careful on the way to not leave citizens alive, an easy one to miss is the hobo guy laying down besides a tent a little bit after the entrance left side.
You must go inside the non-travel buildings (marshal’s office, cloth shop, etc) and kill everybody. The only part you must be careful is the Marshal’s Office, it’s too many guys to storm in. Prepare your team downstairs and send someone to bait them out, easy-peasy.
As you’re getting some fun killing innocent people, there are two other achievements you can easily manage, as there are a plenty of opportunities to do so:
The Badda Bing achievement in Wasteland 3 worth 36 pointsYou blew up 6 enemies with a single explosive attack. The Boomstick achievement in Wasteland 3 worth 52 pointsYou killed 4 enemies at once with a single shotgun attack.
To finish, you do not have to enter the travel points buildings (Little Vegas, Sans Lux Apartments).