A bit more in depth:
Note - if you previously met this score requirement prior to the update, it will automatically unlock upon playing the game. No need to get it again.
You are required to get a score of 600,000. The challenge is a green icon on your map to the left of your Downtown base. Fast travel would be your fastest way of getting there.
This achievement may seem daunting considering a gold medal is 250,000 points, but is actually the easiest in the Title Update DLC. As with the "I should get paid for this" achievement, I would not recommend attempting this until later in the game when your weapons are upgraded and you have access to more amps/overdrives.
Start with your overdrive setup:
• OD Rival
• Weapon Damage – Deployables
• Weapon Damage – Single-shot
• Ammo Capacity - Deployables
• Ammo Capacity – Single-shot
• OD Killer
For weapons, I would choose from the following:
• Acid Sprinkler
• Turret Copter
• TNTeddy (Nuke Explosions)
• Pulse Mine or Proximity Mine
• Hairspray Bomb
IMPORTANT - USE THE SECOND AMENDMENT AMP ON YOUR MAIN WEAPON AND AMMO WILL NOT BE AN ISSUE
Spamming your weapons is the way to go with this achievement. I completed the first 3 challenges up to kill a winger which in order are: Melee Kills, Grind Kills, Bounce Kills. Without completing any challenges, the OD don't increase in numbers and you probably won't end with a high enough score. Plus completing them gives you a nice little score bonus.
Once you have the winger attacking, you will probably notice that there are two main spawns of OD. I focused all my firepower towards the spawn on the left side of the gas station (when facing it). I bounced on a car and occasionally shot turret copters at the other spawn which is on the right of the gas station.
I used the hairspray bomb and turret copters for the duration of this challenge. When fully upgraded, the turret copter will last quite a long time. The combination of both of these weapons got me this achievement on my first attempt.
Please be sure to vote and comment if you give me a negative so I can improve this guide! :)
Edit: some are saying it's better to go an extra round and kill the winger which brings out a herker. I found it easier not wasting any more time looking at the sky than I had to considering the herker charges and throws objects. Give it a try though and let me know which method you liked better.
Edit 2: Clip of me unlocking