If you beat ODST, Reach's Legendary Campaign isn't to hard, just play like you did in ODST and your golden, take it safe and easy, don't try to run and gun to much.
I beat the game on Normal and Heroic, (It doesn't say on my profile because I did one of the missions online and just continued with the campaign from there and some how it did not say I completed it, I'll do it on Heroic again next.
But I just completed "ONI: Sword Base" on Legendary last night and plan on continuing when I have time. I started my Legendary Run on Sword Base cause I already did Winter Contingency on Legendary for a Weekly Achievement a few weeks ago.
Sorry for the TMI, but I felt the need to explain, in case someone checked my profile and saw that it doesn't say I completed the Campaign on Heroic yet.
BTW, LOL at the guy who did not realize you could check his player history here. I really laughed my derriere off, I love it when idiotic posturing @$$holes get caught and revealed for what they REALLY are.
[Edited on 03.21.2012 4:38 PM PDT]