I think you can do the campaign solo on Legendary, there are really only a handful of parts that are really tough, not nearly as many as Halo 3. Just take your time, remember where the health is, and you'll be fine. You'll feel more accomplished if you do it alone, it's almost laughably easy with a friend.
Hardest Achievements: Seriously…, Mile High Club
Yes, I played Avatar, I’m not proud of it, but whatever, if you’re on this site, chances are you did too, or have seriously considered doing it, so :-P