Halo: Reach

You can try and find a firefight game variant online to get this achievement but honestly it’s just easier to make one yourself. Turn Invulnerability on and set your damage to max (300% I think is max) Turn all skulls on and set the difficulty to legendary. You can even take it a step further by setting all the enemies to elites or some other strong enemy. I was getting around 1500-2000 for every elite kill.

Oh and I found it easiest when I turned the dropships off and had a Fuel Rod gun as my starting weapon. Secondary weapon that I liked was the Hammer because it kills elites in one blow.

If none of this is making sense to you and the YouTube link isn't helping I made a game variant that has all of this in it and you can download it in the link below. Just look for the achievements variant and if it’s not there message me on Xbox LIVE and I’ll put it there.
http://www.bungie.net/Stats/Reach/FileShare.aspx?player=INTI...

EDIT: Bungie.net will no longer work for downloading files from other people's file shares but don't fret because you can still download the file. Go to the Halo Reach main menu and hit start on your controller. Now scroll down to "FILE BROWSER" and then go to "NEW CUSTOM SEARCH" then "FILE SHARE SEARCH." Now just type in my Gamertag (INTIMIDATOR363) and once you hit enter my fileshare should pop up.

The following options will get you this achievement in no time, and possibly more.

Set your Difficulty to Legendary (for a score multiplier)

Under Game Options, choose Firefight Settings and Wave Properties and change these settings for Round 1, 2 AND 3 Round Properties. You have to change the settings for EACH ROUND.

Turn on these skulls: Mythic, Catch, Tough Luck, Famine, Tilt, Black Eye, Grunt Birthday, Cowbell, IWHBYD. These skulls will increase your multiplier. Note: You can turn on Cloud if you want, but having radar helps you anticipate enemy drops and score more points.
Now, for each Wave, Initial, Main and Boss, change these settings:
Uses Dropship: Enabled (You will be able to hit clumps of enemies dropping from Phantoms for high scores)
Squad Selection Order: Ordered
All Squads 1-5: Elites (Elites are worth more points than Grunts and you can kill them in one missile hit, unlike Brutes)
After adjusting the settings and Skulls for ALL 3 ROUNDS, enter the Bonus Wave settings and make these changes.

Bonus Wave Duration: 2 Minutes
Uses Dropship: Enabled
Turn on these skulls: Mythic, Catch, Tough Luck, Famine, Cloud, Tilt, Black Eye, Grunt Birthday, Cowbell, IWHBYD.
Squad Selection Order: Random
All Squads: Elites
Back out of the Wave Properties menu and make these settings in the Game Options/Firefight Settings menu.

Wave Limit: No Limit
Turn Count: No Turns
Time Limit: No Limit
Enter the Generator Properties menu and make the following changes.

Generator Count: Disabled
Spawn Order: Sequence
Loss Condition: Lose All
Enter the Scenario Settings Menu and make the following changes.

Hazards: Disabled
Weapon Drops: Enabled
Ammo Crates: Enabled
You do not need to adjust the Custom Skull Settings. Back out to the Game Options Menu and choose Spartan Settings. Ignore the options here -- they won't matter -- and choose the Base Traits Menu.

Choose Shields and Health and makes the following changes:
Damage Resistance: Invulnerable
Immune to Headshots: Enabled
Immune to Assassination: Enabled (This is important -- even if you are invulnerable you can still be killed unless you enable this option)

Back out to the Spartan Settings / Base Traits Menu and choose Weapons and Damage and make the following changes.

*Damage Modifier: 300%
*Melee Modifier: 300%
*Primary Weapon: Rocket Launcher (or Fuel Rod)
*Secondary Weapon: Any (I like the needler)
*Starting Equipment: Jet Pack
*Infinite Ammo: Bottomless Clip (Crucial: No reloading!)
*Equipment Usage: Unlimited

For Loadouts, I would usually just go with the jet pack, and the rockets. You can use any weapon that may help with a daily/weekly challenge, or a needler for the supercombine achievement.

Since you are invulnerable and your rocket launcher will take out Elites in one hit, this just takes time. Your ultimate goal is to just hit clumps of Elites for high scores. After playing for a bit you'll learn the locations of Elite drops. Watch for the drop near the center of the map on the cement platform -- 8 Elites drop here at once! It took about 30 minutes and 750 kills for us to get 1,000,000 points, so 200,000 points should be all the easier.

Go to the Main Menu's Firefight and mess with the settings, making your Spartan invincible. You can even throw on the setting to make your weapons 300% damage multiplied. Just keep killing Covenant until you get the achievement.