I’ve read a post similar to this, but I’m extending this to Reach, Halo Wars, and Halo 3 (not anniversary, since they are all one-player achievable, or ODST, since I don’t have a hard drive, which is required for online play). What this is for is to help everyone get the MP achievements they were unable to get, and want to get all of them before Halo 4 comes out. I’m on quite a lot; what I can be sure of is 11:00pm - 4:00am Pacific Time during the weekends - weekdays vary, depending on how much schoolwork I have to do. I’ll help anyone (in return for the same) any time I’m on. PM me (Archanix029) on Xbox LIVE if anyone want to do this.
This thread is a big help for what you are looking for.
im up for helping with achievemnts. i only have a few halo 3 ones left and all the wars ones left. but i gotta say that “official” thread. it doesn’t help at all. ive commented and mssgd and everythign with many many posts and threads and stuff and only once played with people from the forum for achievements.
cdwumbreon420: Ok so when my internet stops acting up on me, I can message you and find a convinient time. Mainly I’m looking at raising the Reach achievements I’m missing and get it over with, but I’ll help you out with the Wars and H3 achievements.
I’m really behind on the H3 achievements, mainly because it lags on me really bad for some reason. I almost got the Halo Wars MP campaign completion achievement, but my previous member I was playing with on the Wars campaign with left me on the 2nd to last mission - on Heroic, on top of that - and doesn’t want to play it anymore. 
Most other Halo Wars achievements I got, so helping you get them won’t be a prob. And also any H3 achievements you need to get.
Now, I know a lot of the achievements (take the Halo 3 Stepping Razor achievement for example) requires more than two people to get; in this case it would need at least 4. So what I was hoping for is to get a group of players who want to get the achievements, and start a game with only us. That way we all can get the achievements easily, though some may take time for everyone to get (such as the Halo 3 ‘Save This Film’ one). I know you can start 8 player matches with a single party in some Halo: Reach playlists (I came across a group of seven players in one party in the Team Swat playlist once; they mentioned the 8th player quit out). Not too sure of Halo 3, since some of the playlists don’t even let a second person play.