Classic Raider

Please note that these achievements are as retroactive as possible! Quests are actually checked to see if you killed the last boss before. For example, there is a Horde quest to kill the last boss in RFC, so if you completed the quest when WotLK comes out, it will automatically be completed.

This only counts for those quests that REQUIRE you to KILL the LAST boss (or the bosses, if multiple ones are listed).

Another note for the raid dungeons: If you have a piece of gear or other loot off the last boss IN YOUR INVENTORY OR BANK it will automatically be completed. If you got a piece of gear from the last boss and have since then destroyed or disenchanted it, it will not count. This includes things such as the Onyxia Hide Backpack, which I still use to this day. The items rule does not apply to five-mans or heroics.

i luv blizzard now they made an achevment that makes ppl wanna do old time raids :D

Indeed, however it is still possible to have a lot of fun, and face some challenges in the classic raids. We're a group of three friends (Mage/Druid/Paladin) who have been trying this together, and I can tell you, The Temple of Ahn'Qiraj is no joke at level 80 with only 3 people. We actually weren't able to kill C'Thun with 5, but I think it could be doable with 7 or 8 (We gave in, and did it with 11).

As for which encounters that are doable with what, I'd say:

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• Onyxia's Lair can be soloed by some classes/specs. A healer and a dps can do it easily together.
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• The entire Molten Core is probably doable with 2, but you'd want 3 or 4 for some bosses (Majordomo, Sulfuron, Ragnaros). We did all the encounters with 3. Bring someone who can decurse, 90% healing reduction is still annoying.
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• Zul'Gurub up to Hakkar is doable with 2 (Maybe a Paladin could kill a good few of the bosses alone?), but you want 3 for Hakkar so you don't get screwed over by Mind Control.
  
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• Blackwing Lair is hard with 3. Very hard. We used 4 for both Razorgore and Vael (And had to use a combat res on Vael), you still need a Rogue for the Suppression Room, Broodlord still has an ungodly Mortal Strike unless you have an actual tank. The 3 drakes are probably doable with 3 (Maybe even 2, but Ebonroc is going to be hard with only 2). We haven't finished it yet, but we'll probably bring 5 for Chromaggus and Nefarian. The Onyxia Scale cloak is almost mandatory here, as Shadowflame still is very dangerous at level 80.
  
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• The Ruins of Ahn'Qiraj is doable with 2. It's pretty fast with 3.
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• The Temple of Ahn'Qiraj is not possible with less than 5 for a number of reasons. You can do everything up to Twins with 3, though. I wouldn't advice on bringing much less than 10 people for C'Thun, but you can manage with around 5 people up to that easily enough. Skeram was hard with 3, a mind-control of the wrong person meant a wipe.
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Just in case someone wants to get this done with some friends, I hope they can find this information somewhat useful. There are a number of fights you should read up on before you do this though, unless you bring like 10-15 people. Notably Razorgore, Twins, C'Thun.

Update: We tried Ebonroc, Flamegor and Chromaggus with 4 today, and I'd advice on bringing at least twice as many. Ebonroc needs tank-switches at an alarmingly high rate, and is affected a lot by diminishing returns on Taunt. We skipped him, as this was a big problem (He selfheals without tank-switches). Chromaggus has a lot of health, and dishes out a lot of nasty debuffs, it's a very long fight that you should probably have at least 8 people for. We couldn't do it with 4.