Hey,
Man I hate to sound like I'm bragging but I swear on my life that I beat Halo on legendary ALONE in less than a week. The library is absolutely insane but there is certain techniques to pass it fairly easy, corners, thats all I can say in that level, use the corners and let them S.O.B.s come to you: grenade, shotgun, repeat......Now you wanna know the real hard part of halo on legendary?, towards the end when the flood AND the special silver colored enemies all attack you at once. The silver elites are to powerful to battle, so what you do is is run around till the flood is chasing and jumping at you, than you run right towards the silver covenant, than run like hell and watch the fireworks, but make sure you grenade too cause there will still be some silvers left, basically this technique is used as a distraction. And my only other tip for legendary is use your friggin grenades, plasmas that is, you MUST learn how to stick enemies with them at ease, it is the only way to survive. Oh yea, another insanely hard part is on the 3rd level when you get sucked up into the ship and the elites with swords come out and kill you over and over, heres a tip, when covenant come running out of the doors, run inside one of the doors and just post up, make ceratain you have a sniper rifle with lots of ammo cause before you know it about 4 or 5 gold elites come charging you and the only, and I mean only, way to progress past that part is to snipe them at close range...good luck
P.S Spartan, have you beat it on legendary?