This isn't really a guide, but a suggestion for how to go through Legendary. Unless you plan on going for the Lone Wolf Legend achievement as well, having a trusty battle buddy is recommended. Firstly, there are 4 guns you should focus on finding ammo for at all times: Battle Rifle, DMR, Covenant Carbine and Light Rifle. I cannot stress enough about headshots because they will save your life more times than I care to count. The crawlers swarm like crazy, but if you can keep calm and pop heads you will not only conserve ammo, but thin your enemy ranks out really quick.
Secondly, when it comes the Forerunners, after you remove the local crawlers, focus on Watchers and Autosentries floating around because the Watchers will resurrect Knights and Crawlers if you aren't careful. Autosentries will just strip your shields and be a real nuisance when you are out in the open.
Lastly, Knights. I hate, hate, hate, hate Knights. Best solution for taking down a knight (provided its just one) is to distract the Knight for you or your teammate to sneak up behind it for an assassination or a quick blast from the Scattergun. However, that strategy changes based on which class of Knight you are facing. The Battlewagons carry the Scattergun and will down you in one shot and the Commanders will drop you with the Incineration Cannon without hesitation. At this point, the tried and true "Noob Combo" is highly effective at range or pairing up one of the 4 weapons with the storm rifle, which kills shields in no time. Just remember to take your time with Knights and don't just rush at them because if they are caught by surprise early on, they will teleport either behind you or further away to get a better shot at you, which can take your bum rush down real quick.
As with any Halo on Legendary, take your time and get ready to die, A LOT. Take frequent breaks to keep your mind clear and focused so you don't fall into the "Rush everything and hope it works" mentality. Good Luck and have fun!