Like what theEchness said, but I want to give my own tips.
I played on easy first just to get the storyline down before I put time into legendary difficulty. What I noticed on easy was that incinerate cannon or w/e... will kill you regardless. And the Binary Rifle. Both of those will kill you if you take a hit.
My solution to this problem is play with the skull: Catch. Enemies ALWAYS stand still while they toss grenades. They also tend to use grenade over their one-hit wonders. I'm the kind of person who's fine with enemies tossing grenades as it is easy to dodge.
When using this strategy, make sure you have a lot of open spaces, kill jackals, crawlers, and watchers because they don't throw grenades. Be sure you stay at least medium range for your target, that's when they go crazy with grenades and forget their weapons. If you stay around long range, they won't throw grenades but instead shoot you.
Not recommended for players that does not like to keep moving. You'll need a lot of patience. Also not recommended in vehicle-heavy levels.
- Beam rifle does not one shot you.
- Elites do not one hit you on melee
- Binary rifle will insta-kill you (be on the move and the enemy wil lnot shoot, or miss if they do. DON'T run TOWARDS them)
- Prioritize snipers before others, but priortize other enemies if they continuously kill you.
- Aim for the crawler's mouth, not the head.
- If you are going to engage the knights in melee range, make sure you have dealt damage so that you see a little blue on them. Or else they'll survive and kill you.
- Knights' melees one shot you. (Most of the time, at least the regular melee move)
- NEEDLER, BEST WEAPON if you don't have any other weapons to use in medium range. (1 explosion for blue elites and other weak enemies, 2 explosions for gold elites and knights)
- and ALWAYS, slow and steady. Unless you have a shotgun/scattershot and 5 knights with light rifle, then by all means charge in and 2 shot them.
- Halo 4 is very kind with the amount of grenades enemies drop, feel free to throw them at enemies. Just be sure to utilize them fully and kill watchers or keep them distracted so your grenades do what you want them to.
- Average about one hour per level. Even the vehicle map... especially the vehicle map.. man -.-
For those who played Reach on legendary AND completed it, you'll find Halo 4's legendary to be MUCH MUCH easier.
Enjoy the Spartan armor after you finish legendary :D
(My first tips, feel free to tell me the goods and wrongs. I like to help other with their achievements too.)