Sorry man I'll leave that one to the online experts around here. No idea how tough it is.
Here's a link about equipping Invincibility etc. in firefight. One post says you can just do it in the optiions menu without the skulls. I'm in SP mode right now. Later maybe.
http://www.easyxbox360achievements.com/under-5-hours/932-easy-cheat-for-halo-reach-
Looks like it's as easy as you want it to be. 4x speed on Legendary could be nauseating.
http://www.gameguidecentral.com/2010/09/01/halo-reach-achievement-guide/
Here's detailed instructions. Pretty ridiculous how easy some of them are. The amount of customization is just phenomenal.