Looks up Tyrant's videos on Youtube for tips on skipping entire sections. Also, you can load up any of your previously completed levels on the furthest checkpoint available, on Legendary and you'll get Legendary credit for beating it.
It's actually one of the easier Halo games to do legendary solo. Definitely doable, since I had it done just a few days after launch. Sure some other people have it done solo as well. --- IRON MAIDEN IS MY RELIGION! Gamertag: Demigod Elessar (I was formerly Demigod Tyek on these boards, and as a gamertag)
Oh yeah. I'm not going to stupidly say that it's "easy," but certainly doable. There are a few levels that will take you a while but with some patience, you'll get through it. I don't think it's any easier than Halo 3 but much easier than Halo 2 for sure. Halo on Legendary played totally different so it's tough to compare.
Now the jump from Normal to Legendary will be pretty dramatic. I beat it on Heroic first then tackled Legendary. I was surprised that a couple of parts that gave me trouble on Heroic were actually much easier on Legendary because I knew what to do.
You'll really come to HATE the Promethian Knights by the time you're done. --- Five million Cybermen, easy. One Doctor...now you're scared.