If you are viewing this thread I am assuming you need at least one miscellaneous Halo 3 achievement, these include:
- Campaign Skulls
- Marathon Man
- Vidmaster Challenge: Brainpan
- Too close to the sun
- We're in for some chop
- Fear the pink mist
- Headshot Honcho
- Used Car Salesman
- Campaign Meta-Achievements i.e. Guerilla
And any other achievements you need.
I can help you get these achievements fast and easy, I am experienced at Halo 3 and have got these achievements on at least 3 different accounts so I know my stuff. So if you want any of these achievements just reply to this post or send me a message and I'll try to arrange a suitable time.