Probably one of the hardest achievements in the game, but not too bad if you follow the guide below.
Your first and most important step is to complete a good enough portion of the game, up to the point that you have purchased many weapons/amps/overdrives in order to make this easier.
You’ll want the following weapons: (Lvl. 5 with Amps)
• Dirty Harry (Stun Rounds)
• Acid Sprinkler
• Turret Copter
• TNTeddy (Nuke Explosions)
• Pulse Mine OR Proximity Mine
You'll also want the following overdrives: (Lvl. 4)
• Plus/Minus (You need to pick up health with this equipped)
• OD Rival
• Weapon Damage – Deployables
• Weapon Damage – Single-shot
• Ammo Capacity – Single-shot
• OD Killer
Once the above is complete, you'll have to find "Episode #3: Buck's Revenge" located not far from the Fargothian base. You can also fast travel directly to the event using the challenges leaderboard. Before starting the event, refill your ammo! (If you fail your attempt, quit out and refill ammo before retrying)
Once the event starts the order of the objectives is as follows:
Melee Kills (5/10/15)
Grind Kills (10/15/20)
OD Spawner Kills (1/2/3)
Bounce Kills (5/10/15)
Undergrind Kills (5/10/15)
OD Mugger Kills (2/4/6)
Pyro Geyser Kills (10/15/20)
The melee round is pretty straight forward. I suggest using a ground smash in the later rounds to kill as many enemies as quickly as possible after the sequence repeats. Do not jump after your ground smash, this will only waste time. Wait for the animation to end, and then jump afterwards and you can ground smash again right away.
Grind kills are pretty straight forward. Grind the garden railing and use the TNTeddy or Deployables like Acid Sprinkler or Turret Copter.
Next, the OD Spawner can be taken out with the Dirty Harry in 2-3 hits, just remember the 4 possible spawns, use your mini-map and get in range as fast as possible.
Next, for bounce kills, use deployables again like the Turret Copter and Acid Sprinkler and bounce on one of the cars in the middle of the road, near the wire that hangs over the street. After bounce kills transition directly into your undergrind as soon as they are done. Your weapons will do the work for you for this round if you set up the right deployables during the bounce phase. If not, use the TNTeddy here.
For OD Mugger Kills, get on the high rail from the street, and equip your Dirty Harry (or TNTeddy if close range). You'll want to "rush" the Mugger spawns to kill them before they start jumping! The spawns are the same every time although they may move in slightly different patterns. This is the most frustrating round, and the round where you will lose the most time if you're not quick.
As soon as you kill the last Mugger, make a dash for the closest Pyro Geyser at the end of the street near the bridge. Just keep bouncing on it until the kills rack up. If you have TOO MANY deployables up, they may kill enemies before they reach your geyser, but this wasn't a major issue for me.
As soon your Pyro kills are done, transition to the top of the list using a ground smash and repeat as many times as you can.
When you get to the "OD Mugger Kills: 6" round, I suggest you just take your TNTeddy and focus and your attention to the enemies on the street. This round takes a long time to complete, and you are likely to run out of time. You'll score more points ignoring it and going for Kill Streaks on the hordes below!
With all those tips, you should have no problem! I was able to pretty easily get 655,000 without 100% of the upgrades listed above, which is significantly more than the 534,000 required.
Thanks! Hope this was useful. :)