So for those of you that have completed this seemingly impossible task, post here and let your unyielding might be displayed!
 
Also, I'm curious to hear what you think was the hardest part of the game.
 
For me the toughest part was on the last mission, the final standoff before the MAC gun. There were no good weapons, only assault rifles and shotguns, which are completely useless. I was thankful it was Brutes there instead of Elites otherwise my small reserve of DMR would not have been enough. And Emile wasn't around anymore to draw the enemies' attention away.  
 
I was lucky that I accidentally swapped out my Drop Shield for a Hologram earlier because four Brutes dropped in right on top of me (another example of the bad checkpointing system), thanks to their simple minds, they chased my hologram as I sliced them with an energy sword one by one. If I noticed that I picked up the hologram, I probably would have never survived and had to start the level over, which would have sucked because it took me a little over 3 hours to reach that part.  

The beach gave me shit checkpoints randomly and left me defenseless against ghosts. Other than that it was prolly the funnest legendary run I've played
 
(wonder if my achievements have synced up yet....)

Sweet, I'm in too. Just got it this morning. Congrats on snagging those cheevos.
 

i prolly shouldn't be posting in this thread but i am working my way up the chain right now thru legendary. already beat the campaign twice(solo and co-op) on normal and so just jumpin head in and tryin the best i can to stay focused and not get too worked up because of stupid things like the silver and gold elites and friendly ai problems. i'm stuck right now on the night mission. am getting swarmed at the refueling station with drop ships coming in left and right. hopefully i can work it out and beat it soon. hopefully will be able to join ya'll soon in the club.

Can we talk strategy? I'm going to attempt this achievement and i really would like some help with it.

Definitely that last level. Handing the package off & then having to deal with the 4 Elites guarding the mass driver cannon when you have absolutely NO ammo left.

well i had a lot of difficulties with the 2 hunters in sword base the first time in the basement room, because there were only assault rifles and shotguns, and very little room to maneuver around

" Can we talk strategy? I'm going to attempt this achievement and i really would like some help with it. "

Also cheese the HELL out of the active camo and try to steal enemy vehicles. I just snagged a wraith and laid waste to everyone and then used active camo to slip in the control room in the final part of Exodus.

I found the second playtrough on Legendary to be easier than the first one on Heroic,but can someone tell me why my achievements haven't been updated since 9.14?

It was actually surprisingly easy compared to heroic. I've usually played Halo on normal and this time I decided to man up and go heroic. Then I figured I'd try legendary. The hardest part was definately the ending of Exodus (the beach part). Had to destroy the wraiths by stunning them with a plasma pistol repeatedly while running towards them to drop a grenade in.
 
About the brutes/elites, I found the brutes to actually be more annoying enemies than the elites. The elites you could just plasma pistol/headshot while the brutes (especially the grav hammer/fuel rod gun ones) took way more effort. I found the needle rifle to be really effective with only 3 shots to an unshielded target to supercombine. That was especially useful with the shielded brutes, plasma pistol/ 3 needle rifle shots.
 
The thing is, even on heroic you really have to take it slow and be smart about how you deal with the enemies and try to take the elites and brutes one on one. So after that the heroic/legendary jump wasn't really that big since I already had learned the best tactic for each enemy. In fact legendary felt easier than heroic just because of that.

for me definitely "The Package". After handing over the package and fighting 4 more Elites was just... bah... hard? 

" I played the game on normal difficulty setting, and still struggled getting through the campaign. Am I the object of ridicule now? "

Just got through the half way mark. It's been about as difficult as it was in two player co-op which is to say it's probably easier than it would have been trying to do it solo first. 
 
I would say the hardest part for me so far was also the two hunters in sword base before the elevator. I had to circle right and kill the elite and two grunts in the far room where the elevator is before poking my head out of the door to sloooowly kill the hunters off.  

I'm working on this achievement right now. 
 
It burns so good.

Also you don't actually have to defend anything at the defend objectives. Just chill back and let the AI spartans plink the enemies to death.

pro tip: don't bother getting inside the covenant AA guns to destroy them in the second-to-last level, just stay behind the ridge and blast the cannon 10 times with the tank.

I had hella problems with Long Night of Solace. Once you are on the corvette you face off against room after room of elite heroes. Plasma Launcers, concussion rifles (the bane of my existence!), focus rifles, I took a while to get through that. 
 
That last bit was pretty tough as well. Now I'm just waiting for Giantbomb to update my achievements. 
 
Here's a tip that I discovered after completing the game. If you shoot a hunter's weapon as he is charging it up it will backfire and do damage. This would have made my life so much easier! 
 
A typical encounter with an elite on legendary. 
http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=498020&player=Odinez

" The last level was by far the hardest.  I found the battle 4 white elites (glowing red?) preceding the 2 hunters at the end the most difficult due to the weapons I had.  "

The last part was a bitch. Luckily I found a sniper rifle up top and took out the last wave of Brutes from a distance.

I am working on this right now, as my first Reach playthrough. Just completed Exodus. The last part of that mission was incredibly hard. Also, Long Night of Solace. It wasn't as hard, but it seemed as though the enemies just kept coming, like they would never end. Clear a room of 20 enemies, clear next room of 30, clear next room etc...
 
So satisfying to complete missions alone on Legendary though.
 
I managed to somehow screw up though, played through Nightfall, only to realize afterwards that I had played on Heroic. I'll have to redo that one I guess.

I did it with 3 friends, but I haven't tried it alone.  Hell, I haven't tried the campaign alone yet.

I did this last week, and i'd have to agree with your hardest part. That was really the only part i had any trouble on really, until i found that shotgun room. I think i must've gotten really lucky/good checkpoints though because i'm always seeing complaints about the checkpoints in this game but i was getting one like every 5 minutes.

I recently got this achievement after a lot of hard work and determination. Basically I thought the whole Night Fall mission was incredibly difficult. Jun dosen't help you what so ever and there's so much bull shit and not many good weapons.

I did this the first night it was out. Fucking clusterfuck. Dying left and right. But getting it was awesome. I used the AI to distract the enemies a lot.

got it after many deaths, hardest part was probably when i got stuck against the zealot elites in the end forever then finally did a perfect run and got it, but then spent forever on the mac gun

I have this achievement as well, got it a few days ago. 
 
As other people have said, Legendary isn't too hard, you just have to be patient.  I didn't have problems with any specific portion of the campaign, but occasionally picking the wrong set of weapons was what killed me most.  The battle with the elites followed by the hunters and skirmishers towards the end was probably the toughest.  But overall it was a lot of fun.

I just got this yesterday, the Covenant ship was definitely a pain.

I did it, hardest part would probably be the end of Exodus with poor check pointing and the friendly AI broke.

I got this last night! It was generally quite easy for the halo series... but I still struggled in parts. The end of The Long Night of Solace, or whatever it's called, where you have to defend the Jorge, was perhaps the only part where i was getting to the point where I just didn't want to play.
 
It actually stayed fun throughout, which is more than i can say for other games hardest difficulties!

" I did this last week, and i'd have to agree with your hardest part. That was really the only part i had any trouble on really, until i found that shotgun room. I think i must've gotten really lucky/good checkpoints though because i'm always seeing complaints about the checkpoints in this game but i was getting one like every 5 minutes. "

Uh, sure. I thought that it was pretty easy once I got past the 3rd mission (Nightfall) up until the final mission. The waves of enemies that came at you when you were given awful weapons, and having to deal with some elites repeatedly because of bad check-pointing was a pain in the ass.

good tip for Pillar last level: right in the beginning in a shack by the mongoose is a rocket launcher, carry that thing through the ENTIRE level with the occasional swapping for PP or whatever but always bring it with you. Either use it on the 2 hunters or trade it for nearby shotgun, use Hologram and shoot its back and use grenades. Last part, give AI member rocket launcher then get it back once he's dead, use pistol ammo to headshot the brutes or the shotgun, later waves, use the sniper and rocket launcher and poof. 
 
Only issue I had was I was literally running out of ammo with a complete lack of guns against 5 brutes and 3 grunts, went into shotgun room and Keyes showed up and I'm like "oh...nvm then"

Just got it tonight. It wasn't really hard at all if you had a plasma pistol handy. I actually died the least amount of times on the last mission. By then I had figured out how to go about things successfully. 
 
Always had a plasma pistol/DMR/magnum combination on me at all times. Gotta say, the most irritating enemies were the Jackals with the shields for me.
 
Might as well rename Legendary to: Don't plan on progressing unless you have a plasma pistol. 
 
Also, I managed to snag me a Banshee on the defend Halsey's lab. That was a terrible idea because the Banshee controls like shit!

This  really was not that hard.

You're all insane. All of you.

ye i'm on my 4th mission, walkthroughs dont help much as the AI is too cunning, just general strategies.

I got the achievement on my second play-through after I did it on Heroic. I can't recall which level I had a hard time with but what mattered most was to make it to the next checkpoint. Best advice for those still trying to get it done, don't be greedy with your ammo.

Awesomeness confirmed.
 
The hardest part for me was the last part of Exodus. Shitty AI kept stealing my warthog and getting me kill, and the wraiths that seemed to have lock-on. Pretty frustrating. 
 
The last stand was pretty easy. I managed to grab a Needler Rifle, so that made the brutes pretty easy.

I am like 2 missions away, but the hardest for me so far was the flying mission, but not for the parts where you are flying. When I got on the shit I just kept getting raped buy every single wave of enemies in every single room. I think it might have been because I was playing at like 12 at night so I was tired and wasnt taking my time.

Proud owner of this achievement!

Did this in one 11 hour sitting, with break in between every level or two. 
 
I wouldn't say there was any "hardest" part in the whole game. Every level had some point where I had to bash my head against it. Id get through a good amount doing well, then hit a snag, then overcome, repeat, repeat, repeat.

This game whooped my ass harder than I'd like to admit. 
 
But Goddamn me if I wasn't proud as hell to hear that "plink" sound after it was over.

Just got this achievement not 10 minute ago. It was actually EASIER then when i played on heroic. I was alot smarter, adapted quicker and in general tried not to brute force my way through (which is why i died so often in heroic).
 
Hardest bit was the end of exodus. I managed to get a terrible checkpoint with all my marines dead or not moving except for the one gunner in the back of the warthog...WHO COULDNT AIM AT ALL. I ended up getting so frustrated i used the jetpack to jump, bypassing all the enemies and triggering the cutscene...which worked on my first go. Talk about a stroke of luck.
 
It was such a great experience to play it on legendary. Extremely rewarding and made me use guns i normally wouldnt have to (pistol, plasma pistol, brute shot at times). 
 
I cant say how long it took me, but each mission took me atleast an hour. I was going at a total snails pace because i was so afraid to die, but none the less, it worked nearly all the time. Slow and steady win the achievement, right?

A typical encounter with an elite on legendary. 
http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=498020&player=Odinez "

Just joined the club. It's a good feeling: I like it when achievements feel deserved. Next up: getting the Firefight achievements legitimately. 
 
Hardest part? Not sure. The siege at the end took a while, as did the bit immediately afterwards with the four Zealots, but nothing took too long. Proudest moment was doing the entire glacier defence sequence on the first try without dying.

I just got the achievement today. The hardest mission for me was the mission that takes place at night, and has all of those fucking shade turrets. At that point in the game I hadn't gotten a hang of playing it alone on Legendary, so I kept trying to run past everything. If I were to redo it, it'll probably be much easier now that I got some strategies.