Trying to help out TA get its solution database back up to speed.
Credit to rabidkitty4
Download this gametype to your fileshare to get these achievements with ease:http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=21550...
If you are unable to download the gametype from Bungie.net. Then from the Main Menu of Halo: Reach, Press Start> Go to "File Browser"> Go down to "New Custom Search"> then search the Gamertag: "rabidkitty4" without quotes. Then download the Gametype titled "Firefight Achievements".
If you want to put in the settings yourself, refer to this: Firefight Game Settings
The gametype variant lets you have infinite ammo without reloading, infinite health, infinite jet pack, the max point multiplier with Legendary difficulty, and all the skulls turned on. You want to stick with the Rocket Launcher as it is given to you in the loadout screen. This will take you about 1 hour to do this on your own with ease. To get this done faster, get about 2-3 friends to join you.