Always keep an eye out for secret weapons (Incineration Cannon, Rail Gun, Rocket Launcher), Light Rifle is your friend, don't doubt the usefulness of the Autosentry.
I thought Reach was the hardest Halo for me (I never played Halo 2). I'm dreading the inevitable Halo 2 Anniversary's Legendary Solo achievement. From all the horror stories I've heard, I'm sure I'll break my controller.
Same as any other Halo. Find a scoped gun, kill weaklings from a distance, get a shield destroying gun (plasma pistol), take down tougher guys shields, quick change to scoped gun, head shot.
Did it in about 17 hours, however I made it to chapter 5 on Heroic then went back and played legendary from the beginning, wanted to pop all 5 achievements at the same time this go around, lol.
The Knights are a pain, I found the best gun for them to be the Scattershot when you can get it. Also, the sentry thing is a good power as a lot of times they pay attention to that so you can run up and blast em with the Scattershot. Lots of Lightrifle too, they get annoying when they hide and restore their shields though.
I found it challenging but thought Reach legendary was much harder, elite AI was definitely a lot better there. But the fact you can trick a hunter into jumping to his death says it all really. Main issue of difficulty is ammo, and having a decent weapon, sticking to the light rifle and whatever "ordanance" weapon I could find from mission 3 onwards was not fun.
Forgot to add that killing Watchers should always be #1 priority. A Watcher can be a bigger threat than an enemy with a Binary Rifle/Incineration Cannon. Ammo management is key, and all of the Watchers' powers are meant to waste your ammo.
Plus I'll never forget my epic fail when I tried a last ditch plasma stick on a Knight, only for a Watcher to throw it back and stick me...
Part of the problem with ammo is that dropped guns seem to disappear much faster than in previous Halos. It is very likely that Halo 4 sacrificed debris and object count in order to boost other visual aspects of the game.
Being smart about which weapons you use against particular enemies will make thing's a breeze.
As for dealing with knights I highly recommend plasma weapons, needlers, shotguns, or the suppressor as they make very quick work of their shields. Non-sniper precision weapons risk wasting ammo.