If you're a skimmer, the two toughest achievements are actually at the tail end this week, and if you're currently in Icecrown Citadel, there's two currently-unobtainable titles that may interest you here (in addition to another set next week).
Argent Defender (requires A Tribute to Dedicated Insanity under special conditions)
Getting Argent Defender requires you to do Trial of the Grand Crusader on 10-man without padding the difficulty with 25-man gear. But that's kind of misleading, due to the interaction of the 10-man and 25-man loot system in Tier 9. With Trophies of the Crusade available to both ToGC-10 and ToC-25, someone who's been running ToGC-10 exclusively can wind up with a gear set that's almost indistinguishable from (and in some cases, better than) a ToC-25 geared player.
So what on earth can you wear? Daelo, the Lead Encounter Designer, explains here.
Difficulty: If you've already been farming Trial of the Grand Crusader-10, the difficulty of Argent Defender lies mostly in ensuring that each member of the raid is appropriately geared. Otherwise, you just need to proceed through the instance a little more carefully, without the margin for error afforded by better gear. If you're learning ToGC-10 from the ground up, it's a lot harder.
Bane of the Fallen King (requires defeating the Lich King on 10-man heroic) or The Light of Dawn (requires defeating The Lich King on 25-man heroic)
These rather poetic titles are yours for the taking after you manage to defeat Arthas in ICC-10 or ICC-25 heroic. How tough is this going to be? Nobody knows. We only have a little data-mined information on what's going to occur during the Lich King encounter, and Blizzard's been silent otherwise.
Difficulty: Your guess is as good as mine, but it's safe to assume that the last boss of the expansion on heroic mode is not going to be a cakewalk.
Champion of the Frozen Wastes
Champion of the Frozen Wastes is easily within every player's grasp these days, although it was a bit tougher back when Wrath originally shipped. Getting a kill on every end-boss in Wrath heroics will happen eventually through the Dungeon Finder, and if you're sufficiently patient, eventually Malygos and Sartharion will turn up as weekly raid quests too.
Getting a Kel'Thuzad kill will be the real sticking point, as Naxx just isn't run that frequently these days. You should be able to convene a group of like-minded souls to scrape together a kill on some idle weekend, and I believe it's now possible to skip most of Naxx and go directly to Frostwyrm Lair.
Difficulty: Not difficult so much as time consuming, given that tier 7 content has been outclassed by badge gear.
Champion of Ulduar (10-man) and Conqueror of Ulduar (25-man)
After the RNG nightmare that was The Immortal in tier 7, Blizzard changed the equivalent achievements in Ulduar to evaluate your "immortal" status on a boss-by-boss basis. That is, if you go through the instance with clean kills on Ignis, Razorscale, and Hodir, but lose people on Freya, Mimiron, and Thorim, your "immortal" status still counts for those first three bosses. So you'll just need to focus on keeping people alive through any boss you don't yet have an "immortal" on the following week, and the week/s after that if necessary.
Unfortunately, this does mean that you usually kiss any other achievements that week good-bye, because there are some achievements (like I Choose You, Steelbreaker) where people have to die, and other fights (like Firefighter) where not losing someone would be a small miracle.
Difficulty: There's still a big heaping dose of RNG to Ulduar "immortal" achievements -- it just can't be helped if somebody goes offline on Mimiron, natch -- but these are a lot more manageable than their Naxx equivalents.
Conqueror of Naxxramas (requires Realm First! Conqueror of Naxxramas)
Unless your server is really behind the times, there's no way to get this anymore; it's only available to people who participated in the first 25-man kill of Kel'Thuzad per server.
Difficulty: This depended a lot on the individual raid. Quite a few realms had multiple guilds already familiar with Naxxramas and able to level in Northrend with the benefit of Sunwell gear. Consequently, getting the Realm First! titles for tier 7 boss kills became a race to see who could get to 80 and field a working raid the fastest.
Death's Demise (requires Realm First! Death's Demise)
Yogg plus 4 keepers right now is extremely easy, especially with the benefit of badge gear. Yogg plus no keepers right now is...slightly easier than it was, but phase 3 is still a horror. Losing Freya's Sanity Wells hurts. Losing Mimiron's Destabilization Matrix really hurts. You'll understand the value of Hodir's Protective Gaze in the moments immediately following a one-shot from a Crusher Tentacle while under the effects of Malady of the Mind. But it's losing Thorim's Titanic Storm that turns phase 3 into an absolute screaming, gibbering nightmare, where an ever-increasing number of completely unkillable Immortal Guardians spawn and will heal Yogg unless you can get them away from him in time. If that's not enough, you also had to beat everyone else on your server to the first kill.
On the plus side, the reward for being among the first people on your server to do this was a Lovecraftian title inspired by the famous couplet from "The Nameless City:"
That is not dead which can eternal lie,
And with strange aeons even death may die.
Difficulty: Is "absolute screaming, gibbering nightmare" not sufficiently descriptive?
Grand Crusader (requires Realm First! Grand Crusader)
Anub'arak on ToGC-25 is no joke, even now. Your raid faced (and on some realms, still faces) three difficult and time-consuming hurdles:
- Gearing a tank or tanks in what's usually referred to as "the +block set" for Anub'arak's adds. Bears and death knights need not apply.
- Getting everyone in the raid comfortable with kiting Anub's spikes during phase 2 for as long as possible. You know "that guy" -- the one you prayed didn't get Shadow of Death on Teron Gorefiend, the one who couldn't kite Doomfire on Archimonde, the guy who died on every Thaddius and Heigan kill? He's still in your raid now. Have fun with that!
- Phase 3, where the encounter gets really hard, and your healers have mere tenths of a second to react to Penetrating Cold targets, all while keeping themselves and the raid hovering at just a sliver of health while the tanks are getting crushed under a mass of bugs.
- The title was only for being the first raid on your server to get to Anub'arak with 50 attempts remaining (i.e. one-shots on all four preceding ToGC-25 bosses), and then one-shot him as well.