Achievements are used to mark difficult (or sometimes pathetic) challenges in video games.
Some achievements are somewhat simple, such as finding an Easter Egg or completing a level. Others can be irritating, such as getting a certain number of kills within a short time span or things like that. Others, MAKE YOU WANT THE THROW THE CONTROLLER IN A FIT OF RAGE. These achievements are very tedious and involve split second timing, fast reflexes, and perfect aim. Post your achievements that you think are difficult to obtain.

2 come to mind. 

Call of Duty 4- Mile High Club
Anyone who knows me fairly well on this site knows I am not a fan of Call of Duty games. I think they are super overrated. I get bored of them and I think they are super easy.  But the Mile High Club achievement is one of the hardest ones I have ever earned. At the end of the game, there is a level on a plane that involves a hostage at the end. You have to go through 2 whole floors of the plane. Both floors run through a pretty good length of the plane (about 3 long narrow rooms each). Throughout the level, you have to shoot through over 25 enemies. Keep in mine, that is 25 enemies all shooting at you while cramped into a total of about 6 narrow rooms. On top of that, you have exactly 1 minute to get through all of them, or the hostage dies. There are no respawn points on this short level (which I like, by the way). You screw up, you start ALL over again. On top of all that, you have to do it on Veteran difficulty, where a mere 2 consecutive shots pretty much kill you. This was frustrating beyond belief, but one of the most satisfying achievements to earn. You feel like a complete bad*** when you finally get it.

Mega Man - Mr. Perfect
We all know the classic side scroller Mega Mans. They are great, and almost all of them give you a pretty good challenge. This Mega Man is completely true to the NES game play and difficulty to the T. But here is the kicker. You have to beat the entire game taking no damage. Let me rephrase that. Beat the ENTIRE game taking ABSOLUTELY NO DAMAGE. That was, hands down, the biggest pain in the but achievement to do. So many times, I was close to finishing the game, took damage, and had to start all over again. It made me want to throw my controller through the screen.

Those are the 2 I think are the most frustrating. 

100% Battle Trophy Collection in  Star Ocean: The Last Hope. Takes literally hundreds of hours.

rcarter2 :
I did that first one. Just for fun though, cod didnt have ps3 trophies.

I cant specifically recall the hardest one iv done.
I know completing mass effect 2 on the hardest difficulty made me want to put theplaystation through the window, other than that i can only think of lead asprin in resident evil which required a mid air headshot. To be fair i actuallly got it accidentally when a guy jumped at me and i hit him in the head with an arrow of all things. Id given up on ever getting it.

These are just some achievements that I did by myself with no motivation whatsoever other than boredom.

SSBB - Very Hard difficulty with 1 life with no continues

One life. No continues. Pretty gutbusting and make you want to throw the console out of the window. I beat the game without taking a single death, and if I did, I had to begin from the beginning. I finally did it after using Link, one of the worst characters. I was pissed.

UMvC3 - No damage in an online battle

Yes. No damage, at all. And greenhorns didn't count. I tried to do it, but failed every time. I finally did it when some dude went away from the console or something, so I beat him with Phoenix Wright.

I can think of some, though the 2 I've had the most problems are with The Binding Of Isaac.

Platinum God:
Get every item. EVERY SINGLE ITEM. This could take a couple of days to a whole year. this also includes beating the game with every character, multiple times and getting every secret (achievements) in the game. This isn't really THAT hard, just annoying and takes a long time.

Cave Boy, Spelunker Boy, Dark Boy and Mama's Boy:
This are different achievements, but they are the same thing, Beat certain floors without taking NO DAMAGE. The last one requires you going down 8 floors without taking damage and considering how hard this game is, this achievements were actually the last I got, besides Platinum God

That is all that I can think about for now.