There is an achievement for solo beating Reach on Legendary. I'd still help you complete it in cooperative play, but you'd miss out on that other achievement.
I'm not particularly good at Halo but hopefully I wouldn't be too much of a drag in a co-op group. I'd love to finish up the Reach and ODST campaigns on Legendary co-op with a group here.
Also, if people are looking to get them in Halo 3, you'd probably want to be in a party with people. The multiplayer isn't dead, but it's somewhat rough to get into games, especially since it seems like most other players are in parties already. Lone Wolves probably isn't hard to find games in, but anything outside of that might be difficult. I tried playing Action Sack a few times in Halo 3 over the weekend and the matchmaking process took several minutes.
I've pretty much stopped playing Halo: Reach's multiplayer as the last group of competitive games I've played my entire team quit out repeatedly. 4vs1 or 8vs1 made me begin to question why I'm paying for Live. And Firefight is just full of high ranks idling to get their perfections up. It's like nobody actually plays anymore. I'm sure other people have different experiences, and I just have bad luck, but it takes me out of the game.
If people are looking to play, I still have all of the games, and would help you get what your after. I just don't want to boost for things, as I like getting the achievements in Halo and saving the videos of them.