Don't get me wrong -- if you have the group for any of these techniques, don't let me stop you. But the kamikaze approach doesn't require much beyond a callous disregard for anything but the achievement, and that attitude that will serve you well in your pursuit of the drake.
Have your tank run straight for the boss, aggroing everything along the way, and then blast the motherloving snot out of Krik'thir, ignoring everything else. Yes, you will die doing this (don't release!), but if your DPS is good, Krik'thir dies with at least one group member still alive.
There you go. That's the strategy.
Having tanked this on multiple occasions as a Druid, I think a shield tank would have a slightly easier time with it, as block would more effectively mitigate the damage done by the smaller adds. Bears also aren't the best pick for AoE threat situations, and Priest healers will have difficulty due to Watcher Silithik's AoE poison ability, but Krik'thir should die so quickly that none of this really matters. Bloodlust/Heroism, as always, is a big help, but I've done without it.
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Anyway. Understand that this is for educational purposes only, that I have not personally suggested this technique to any of my groups, and that this loathsome and utterly repugnant practice was foisted upon my unknowing self by my friend's cousin's sister's boyfriend, who is probably in the Mob or at least deserves to be in jail for something else. The hell of it is, it works.
Hadronox Denied is most efficiently done with a Hunter, Rogue, Mage, or Night Elf in the party using Feign Death/Vanish/Invisibility/Shadowmeld to reset the encounter after aggroing the first mob pack. After the player drops threat, all mobs will despawn. Run down the hall toward Hadronox's spawn point and wait for him to reappear. When he does, you will want a repeat of your performance on the Suicidal Krik'thir Kill-A-Palooza.
There's essentially a soft enrage with this strategy (insofar as it could be called one) because you will eventually be overwhelmed by adds, some of whom will die and heal Hadronox if they're killed by Leech Poison. If you had the DPS to pull off the Krik'thir zerg, however, you can almost certainly manage Hadronox. Now go do some community service to assuage that conscience of yours.
This used to be a 2-minute Anub'arak kill and thus an insanely difficult achievement that usually required a 1 tank + 4 DPS setup, full raid consumables and as many buffs as you could manage, a Bloodlust/Heroism, massive burst DPS, Slow/Curse of Tongues/Mind-Numbing Poison, and no small amount of luck. Blizzard wound up nerfing the achievement to 4 minutes after arriving at the conclusion that by the time you finished filling the group with the buffs you needed to pull it off, there wasn't much room left to get anyone else the achievement.
As of now, this almost inarguably the easiest achievement in the instance and one I've managed with PuG DPS. Have your DPS open up quickly on Anub'arak, and have your tank ignore the nonelite adds on Anub'arak's submerge phases in favor of buliding threat quickly on the elites (any DPS or even a healer can kill the nonelite adds without issue). Get the elites down quickly while Anub'arak is submerged, and you have the fight.
That takes care of both heroics in Dragonblight. As we're doing this alphabetically, we'll continue to --
CULLING OF STRATHOLME
The two achievements here aren't overly difficult compared to elsewhere, but unlike most achievements outside of Oculus, you shouldn't realistically expect to get both of them done in a single run (at least, not at our gear level). With people in, say, Icecrown gear absolutely mowing through this instance toward the end of Wrath I can see it happening, but until then, plan on two runs.
The Culling of Time
You get an achievement in addition to a shot at a really nifty bronze drake mount while doing a timed run. These runs become much easier with practice, so don't angst if you don't win the roll the first time around.
I should probably devote an article to the Stratholme speed run on its own, but suffice it to say that so much of Culling of Stratholme's time expenditure is pre-determined by the instance's "script" that there really aren't too many tricks to doing this achievement. You need good DPS, a tank who won't threat-cap them, a healer who's not prone to running OOM, and you need to keep pulling as quickly as possible. If one of these four requirements can't be met, odds are good you'll miss the timer. The following may help, however:
1. AoE DPS is useful but not necessary if your overall group damage is still high without it.
2. Don't stop to drink until you absolutely have to, and preferably not until after bosses. Bring mana pots and keep running from pull to pull.
3. If you've previously missed the drake by only a little bit, ask everyone to use consumables.
4. Have your tank use a marking mod to quickly mark a skull target on pulls. Ideally, they should be in their threat set; trash and even boss damage are not high here, and he/she can always switch to a mitigation/stamina set for Mal'Ganis if they'd like.
5. Healers should prioritize higher spirit/mp5 pieces over raw +spellpower output.
Ultimately the best and most important piece of advice for these runs is simply to keep moving. There is an element of luck in spawn points on the trash waves, but it doesn't wind up playing a huge role in whether or not you make the timer.
This achievement is a lot of fun, but again, don't try it while doing a timed run unless you want to give yourself an ulcer.
The zombies you'll encounter in the first part of the instance (i.e. before you and Arthas hit the town hall) do not quite reach 100, but you'll need all of them in order to pull this off. The key to doing this achievement is that these zombies will continually respawn until after you've defeated the second boss (Salramm the Fleshcrafter). You can play through the instance normally killing as many or as few zombies as you want until that point.
When Salramm spawns, have your tank pick him up and drag him into the town hall. To give yourself a better sense of the zombie respawn timer, kill the zombies immediately in front of the building, and then keep Salramm tanked inside without killing him. When the zombies in front of the town hall have respawned, it should be safe to kill Salramm; that means all of the zombies elsewhere should be up.
Continue with the instance normally until after you've defeated the third boss (Chrono-Lord Epoch) and had Arthas unlock the secret passageway. Have everyone but your tank and healer accompany Arthas downstairs to the street below, but do not speak to him again. Your tank and healer should backtrack to the first part of the instance, with the healer waiting at the entrance to the town hall and your tank running around aggroing all of the zombies. Use no-damage threat moves (I use Demoralizing Roar, for instance) to build and hold aggro, and turn off any reflective damage buffs or abilities you have. Because the zombies don't run very quickly, you should have a good head-start on them by the time you get back to the town hall. Heal up if you need to, and then make a beeline for the rest of the group.
While the rest of the zombies are slowly making their way through the town hall, aggro and tank (but do not DPS) about 15 more zombies from the street below. When the rest of the pack catches up (and watching 80+ zombies descending the stairs towards you is one of the highlights of the Wrath 5-man tanking experience) and gets within range -- unload. 100+ zombies should die within seconds, hundreds of numbers and spell graphics will suddenly flash on the screen, everyone will go offline but that bastard with the really expensive computer, and you've got yourself another achievement.