This may come as a surprise for most of you but I am a huge Halo fanboy; I've been hooked on the game since its very early stages and am also a keen reader of the books (yes there are books based on Halo). Heading straight to the subject of this blog I'm going to be explaining the basic strategy on how to beat the game on Legendary (this is for all those so-called "noobs" who can't).
The first tip I should share is to be patient in your game play; don't go rushing out into the battle guns blazing and grenade throwing because no doubt you WILL die. Halo campaigns are designed for you to have huge firefights, with large numbers of enemies shooting at your backside from all angles. However, this doesn't mean you should camp either, as most enemies will know where you are and rain down on your location, so it's best to run in and out of cover. Taking it slow through certain areas allows you to find cover and also find some key weapons hidden in the area. On Legendary it's a good thing to kill all enemies before you proceed into the next area just in case they come back with a vengeance!
The second tip I will be happy to share is to pick off the weaker enemies before you work on the bigger ones, for example kill the grunts before you kill the elites; this is because on Legendary the bigger enemies take a lot more to kill, and when you are busy focusing on them you don't want little grunts coming up behind you and tea-bagging your ass. But if you don't have an objective in a certain area feel free to find a vehicle and drive your way out of there.
Never ever let any AI drive your vehicle, they are like old women when it comes to it, smashing and crashing in to rocks, but if you have a spartan member with you use them as a meat shield as they absorb bullets like bounty absorbs orange juice.
If you have the opportunity, get the sniper and keep it! Don't waste it on grunts and jackals, save it for the elites and hunters, brutes are easy with a DMR so don't use it on them either. The sniper is the key to surviving in legendary, remember this!
Another tip is to not pass up any opportunity to use the plasma pistol! I know it's nooby online but seriously when it comes to legendary and against some elites it's quite effective. Well, to be honest plasma weapons are something to always have on you as they can tear through elites shields better then any bullet based weapon.
This tip is for if you're playing with 4 players, leave your noobiest player behind, let him drop back while the rest of you kill everything that moves, this means if any of you die you re-spawn on the noob in a nice safe area.
My final tip is to prefer long ranged weapons, killing enemies from afar is much better as you run less risk of getting beat down by raging brutes and avoid the hammer fist of the hunters.
These are but a few of many tips I could share but I believe these are the most important and hopefully you will all be able to see the master chief Easter egg at the final mission end cut scene, oh yeah and you'll gain some valuable achievements. :P