Score 50,000 points in a Firefight game.
Blaze Of Glory
Score 200,000 points in a Firefight game.
Heat In The Pipe
Score 1,000,000 points in a Firefight game.
Earn a Killionaire medal in Firefight.
Game, Set, Match
Complete a Firefight set on Legendary without dying.
Unlocking these achievements is a cinch if you use a custom game variant. Do note that while using this method won't get you banned, it will remove any challenge (or tedium, depending how you look at it) derived from these achievements.
Below is a link to a custom Firefight variant you can download and use to easily and quickly unlock the Firefight achievements listed above:
To download, ensure you're logged in with your main Gamertag and then click the Download link in the bottom-left corner of the description panel to add the file to your Halo: Reach active transfer list. The next time you start up the game, the file will automatically download and then be available for selection.
To play the variant, select 'Firefight' on the main menu, then enter Gametype and select 'SuperCheats.com' from the list. Select the Corvette map and set the difficulty level to Legendary. Don't worry, as this variant renders your Spartan invincible and your enemies frail.
You can rack up a huge number of points in a short period of time, especially once you reach the Bonus Round. You can likely hit the million point milestone in about 15 minutes, and nabbing a Killionaire medal during the Bonus Round is a cinch.
Ensure you've set the difficulty level to Legendary so you'll unlock the 'Game, Set, Match' achievement as well.
For best results, rip up one of the mounted Plasma Cannons on the raised platforms at the north end of the area where the Elite Generals spawn.
Check the video below to see the SuperCheats.com Bonus Round in action.
Custom Firefight Variant Bonus Round
This variant can also be used to complete certain daily challenges. The recurring Shootin' and Lootin' challenge requires you to kill a certain number of enemies in any game mode in Reach. This includes custom Firefight, so load up this variant when this challenge appears, set the map to Courtyard on Legendary and Jetpack around with your Rocket Launcher to rack up an impressive kill count with minimal effort.
Score 15,000 points in Score Attack Firefight Matchmaking.
You'll have to play Firefight Matchmaking to unlock this achievement. Make sure you're using the Score Attack playlist. Earning 15,000 points in 15 minutes is an easy feat, especially if you choose a mode like Sniper Fight. Bring a friend along for an even easier time.