The previous solutions should be more than enough to assist you in completing your solo legendary play through, but I'd still like to offer my own take on the matter. Firstly, I consider this is to be by far the easiest Halo to complete compared to the previous instalments so if you've completed previous Halo games on legendary solo, you shouldn't have any trouble completing this one. For those of you who are new to the Halo franchise or have never attempted a solo legendary play through; you may find this to be somewhat challenging, but nothing that you can't overcome with patience.
You'll find rifles (DMR, battle rifle, covenant carbine and lightrifle) to be your greatest ally on the battlefield. They offer the accuracy, distance and power required to defeat the various enemies you will encounter. Because of this fact, you should always be sure to have one of these weapons in your possession. Other weapons that may prove useful to you include: boltshot, plasma pistol, scatter/shotgun and the various launchers such as the rocket launcher. The boltshot can take down the shields of Knights and Elites, allowing you to easily dispose of them with a melee or headshot. Plasma pistols are useful for disabling vehicles so that you can approach and board or destroy them. The scattergun and shotgun offer a similar usefulness as the boltshot while offering more range and power. Launchers are useful because well, they go boom. Every weapon has its uses so see what poses to be the most efficient and effective for you and go with it.
As for a play style, you'll want to maintain distance from your enemies. This is why I've recommended the use of a rifle seeing as rushing into mobs of enemies will generally result in your death. By keeping your distance from enemies, you will be able to keep all or at least most of them in your sights while also being able to retreat to cover more easily when necessary. This brings me to mention my next point. Cover. Be sure to take cover whenever your shields are red because you are very susceptible to death at this point in time. To insure your safety try to keep near objects that can offer you a decent amount of cover from enemy fire when the situation calls for it. As for armour abilities, I did not find much use for them. Every once in a while I'd make use of the hologram to give me a few seconds to move in closer to enemies to use my scatter/shotgun or boltshot. I also found the hardlight shield to be useful to move to cover while reflecting enemy fire. Besides that I did not make much use of armour abilities. Furthermore you should aim to eliminate weaker enemies before setting your sights on the larger enemies. This is because larger enemies generally do not pose as much of a threat alone or even in pairs or small groups, but with the additional fire power of smaller enemies; the situation changes drastically. Smaller enemies are extremely easy to dispose of even on legendary difficulty with a single headshot. Approaching and disposing of the larger enemies will be quite the simple task once the smaller enemies are dealt with.
Completing this game on legendary difficulty will be a walk in the park for Halo veterans, but may prove challenging to any newcomers. However, this challenge can be overcome by applying the above mentioned suggestions. Be sure to use weapons that prove to be the most efficient and effective for you. Take cover from enemy fire whenever your health is in danger. Focus on disposing of smaller enemies before setting your sights on the larger enemies in each location. All in all, Halo 4’s campaign is enjoyable so don’t forget about having fun during your legendary play through!