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.
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.