What is the hardest achievement in Halo: Reach. For me the achievements weren't really all that hard. I played the Legendary campaign at a very meticulous pace, so only had to retry a couple of sections maybe 10 times. Even the beach mission, I only restarted twice. I think it is because the game allows various tactics to a varying levels of success. It is very rewarding, but not once did I have to throw my controller. Which is the true sign of a hard achievement. (Shank on Hard). What was your experience like?

I don't know if any of them are particularly hard. 
 
The assassination on the elite one is probably the trickiest as it seems to be a bit hit or miss as to whether it will work or not. 

I tried the achivment where you uasassinate an elite before dying to death but it was hard as fuck. I couldn't get the assainste to trigger, I just kept dying. After 45 minutrs or so i just said screw it ona nd went on with the campaign. Legendary ain't too hard on co-oip with the rspawn stuff, probably hard as hell on sol o0thug. 

" @Hourai: Yea it is punishing if you ever rush. You have to camp and take peak shots at the grunts. and then lure the elites out. Kill the grunts. Always. "

Hardest Achievement? 
Easily "Make it Rain" which is for buying a item from the armory that requires Lt. Colonel.  
It will take me forever!

Legendary Solo is manageable, but definitely requires some skill.  If you're not good at the game, you're probably not going to be able to get it.  Purchasing a Lt. Col. item isn't exactly difficult either, it'll just take a very long time for most people, but that's how it's supposed to work.  The ranks are supposed to encourage you to keep playing the game.
 
I'd say the hardest one is getting 1 million on firefight IF you're doing it fairly.  It's rather sad how they're rated as common achievements now, even though in ODST very few players broke that kind of score.  There really should have been a standard firefight match up, the same as what we had in ODST that you couldn't customise.  It could have been used for earning cR and these achievements.

" Legendary Solo is manageable, but definitely requires some skill.  If you're not good at the game, you're probably not going to be able to get it.  Purchasing a Lt. Col. item isn't exactly difficult either, it'll just take a very long time for most people, but that's how it's supposed to work.  The ranks are supposed to encourage you to keep playing the game.
 
I'd say the hardest one is getting 1 million on firefight IF you're doing it fairly.  It's rather sad how they're rated as common achievements now, even though in ODST very few players broke that kind of score.  There really should have been a standard firefight match up, the same as what we had in ODST that you couldn't customise.  It could have been used for earning cR and these achievements. "

Also completely forgot about "they came to hear me beg".  Seems a lot of people had trouble getting this one, but it didn't take me too long.  You just have to jump off and assassinate.  The tricky part is that sometimes if you're still sprinting, the fall won't kill you and thus won't trigger the achievement.  But at the same time you have to be going fast enough to get enough distance to hit the elite.  I also just bopped him over the head and killed him once or twice without triggering the assassination.
 
It takes a bit of practice but it only took me 10 minutes.  I think the number 1 problem most people are having is...  achievements don't unlock on easy!  Don't replay the level on the easy setting!

Going through the game on solo legendary while the house is working out the kinks with the broadband (they be stealing my bandwidth) and it's been mostly enjoyable. I say mostly because I stopped having fun during almost all of Tip of the Spear. Starting you off with with very little cover and a lot of overloaded plasma pistols against you isn't fun the first time. I found that entire mission a drag, but the rest has been really enjoyable. There's been a few "Oh come on" moments, but it's nowhere near as bad as I anticipated it.  

Legendary on solo, 
I blew through solo on heroic my first playthrough in 2 sittings, 
But Legendary solo is quite the grind.

Legendary was the easiest Halo Legendary yet....so not that.
The elite assassination was easy enough...
The 1,000,000 points can be done with invincibility....
 
I guess the only hard one would have to be the longest one, get to Lt. Major rank

If it wasn't for eventually getting lucky I'd still be without the achievement for assassinating an elite while falling from a height that would kill me.  I spent about 20 minutes and was about to give up when it finally popped after every other attempt ended in a falling death or late melee hit that didn't count as an assassination. 
 
Next hardest will be doing legendary on solo as I've finished the first two levels and both were excruciating.

Maybe I just suck at halo, because Im struggling through the campaign on heroic. One day I'll beat it.  *Fingers Crossed*

for those of you struggling in the campaign: you ARE using the noob combo right?
 
as for the achievement, only one I'm missing is Lt Colonel and I still say assassinating the elite was the toughest. I did it 9 times and nothing, once got it and I'm like "so you couldn't do that the other 10 times or what?"

I didn't find Legendary solo very hard at all up until the ending portion of Nightfall (or whatever the sniping mission at night is). I had the occasional death, but that was only because of my stupidity. But now, this section has killed me AT LEAST 70 times now. I try a different approach every time too. It's the section where you must scale down the cliff right at the end, and there's a turret across the way shooting at you and tons of Elites who just bum rush. It's like impossible to take that section cautiously too, because if you don't look over the rocks, you don't even have a shot at anything. But if you DO look over the rocks, you have that turret killing you in two seconds. The plasma pistol makes it stop for a little bit, but before I'm able to destroy it it kicks back up again. The closest I've gotten to beating it is I scaled the rocks behind it and took out all the grunts, but then the Elites just rush you anyway and you have no chance. And your partner's AI is HORRIFIC.

Legendary Solo, like reaching Lt. Colonel, is just a slow grind. TBH, there isn't a massively hard one.

Beating the game on legendary solo.

I'd say the captain/ Lt. colonel ones, mainly because those are the only ones i don't have lol. The other ones really weren't to bad for me, these ones just take a long ass time.

The only ones I have left are the Captain rank, the Lt. Colonel rank, and the Invasion one (mostly because I dont' usually play this). 
 
The killing the elite while falling was definitely the hardest. Well, I mean I assassinated him about 100 times, but the thing just wouldn't pop. It literally took hours.